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THE POWELL & LEALAND No 1

MAKER: POWELL & LEALAND

DATED: 1878

SIGNED ON THE ARM: 'Powell & Lealand, 170 Euston Road, London' and also dated '1878'

INTRODUCTION DESCRIPTION ACCESSORIES HISTORY

Please Click On Any Picture for a Larger Version

NOTE:This microscope is not in my collection. Most of the text, images, and information provided on this page was kindly provided by Dr Jurriaan de Groot to whom I am indebted, not only for the information and images supplied on this page, but also for being a good friend.

INTRODUCTION:
This page is about an example of the famous Powell & Lealand No.1 stand, regarded by many as the ultimate example of the microscope-maker's art and skill, the Aristocrat of the Victorian period of brass and glass.   This particular example is unusual, in that it comes with a comprehensive and virtually complete set of accessories, including almost all of those listed in the firm's 1875 catalogue. This would have cost its original owner the then considerable sum of £264 7s, (about $27,500 (US) in 2018). Even without the accessories, the P & L No.1 was so expensive, that only the wealthy could afford to buy one on their own.

DESCRIPTION:
This microscope arises from a massive English Tripod foot with a span of about 7 x 9 inches, with each toe of rectangular profile. The body is suspended on trunnions. The trunnion support is close to the table, allowing for a low center of gravity, which is necessary to produce a high level of stability when the instrument is positioned in either a highly inclined or horizontal position. The microscope would be placed in a very inclined position for direct use of an oil lamp for illumination (without the mirror) during use of high powers, and horizontally for drawing using the camera lucida, bringing the eye of the observer to about 10 inches (250 mm) above the table, this being considered equal to the point of comfortable near vision.

sigdateCoarse focusing is by straight cut rack and pinion, which is sprung, and can be adjusted for wear. The rack is attached to a solid bar which has a trapezoidal shape in cross section. Attached to this bar is the arm which holds the body tube. Fine focussing is by a micrometer screw acting on a long lever running through the arm, and acting on the sliding nose piece, whereby one turn of the screw represents 0.16 mm of vertical displacement. The microscope is signed on the arm: 'Powell & Lealand, 170 Euston Road, London', and dated just behind the fine adjustment screw, '1878'

stage understage The beautifully finished Turrell stage is operated through concentric knurled knobs, allowing for a movement of 1 inch across the X and Y axes, which can be tracked on specimen-finder scales graduated 0-100. The stage can be rotated through a full circle, graduated on a silvered scale 0 – 180 – 0 by 0.5 deg. The rotating motion is operated through a conical rack and pinion movement with a diagonally placed knurled knob.

substage PL #1The substage mount(left), is focussed by rack and pinion, is centered by a pair of screws set at right angles (range about 3 mm for X and Y axes), and features full circular movement, activated by a diagonally placed knurled knob. We should note there is no substage fine focus, as this feature, to facilitate precise focusing of the condenser at high power, was not added until 1882.

mirrorThe large plano concave mirror(right)is 3 inches in diameter, and is on an articulated arm attached to a collar running on the tail piece of the lower limb.

The body tube is a Wenham type Binocular; it can be used as a monocular by removing the prism housing and replacing it with a brass insert that contains no prism, but serves to keep out dust and stray light. It may have originally been provided with a separate monocular tube, though none is present now. The Wenham binocular tube can be extended with a rack and pinion movement from a calibrated 0 to 2 inches, allowing for separation of the eyepieces, to adjust for interocular distance, but also for tube length, if used as a monocular. Amongst the accessories is an erecting tube, which was meant to be screwed to the bottom end of the monocular tube, to reverse the upside-down, and left-right image effect of the compound tube, allowing the microscope to be used at low powers for dissection.

prismsThe binocular body tube can be fitted with one of two different prisms, each contained in its own maroon leather case, lined with blue velvet, and white silk inside the lid. The first is engraved For Low Powers and Opaque objects. This is of the classic Wenham design, producing stereoscopic vision through a single objective with focal lengths down to ½ inch, and an angle of aperture not much more than 50 degrees. The other is engraved Powell & Lealand No 105 patent. This features a prism arrangement of their own design, intended to produce binocular non-stereoscopic vision at high powers of magnification. Inevitably with this arrangement, the image seen with the left eye is much fainter than the right.

in caseThe No 1 comes in its original nicely finished original mahogany case measuring 280 (w) x 328 (D) x 460 mm (H), in which slides a mahogany accessory case in a vertical position on the right hand side. This measures 300 x 324 x 85 mm. The uprights in the case are lined with a mid-green baize. The case also contains a bulls-eye condenser on stand, and a tommy bar for adjustments. There is storage facility for a monocular tube, which unfortunately is no longer present. The whole ensemble weighs some 20 Kg. The door is of simple solid unframed construction.


ACCESSORIES:

EYEPIECES:
eyepiecesThe eyepieces are of a large diameter (1.43 inch = 36.4 mm), the same size as the push-in fit for the substage mount, so that any eyepiece can be used as a condenser. The achromatic Kellner eyepiece was recommended for this purpose in particular. Eyepieces include 3 sets of twin eyepieces, No 1 (5x), No 2 (7.5x), and Kellner (Orthoscopic 10x). One of the No 1, and 2 eyepieces each is marked “R”, whilst the No 1 “R” is fitted with a moveable pointer. In addition, 3 single eyepieces are included: No 3 (10x), No 4 (20x), and No 5 (30x), all on the Huygens principle. A Ramsden eyepiece micrometer is also included, made of heavy lacquered brass. It has a wheel, which is graduated 0-100 parts, and is activated by a knurled knob. It seems to be in as new condition, with both wires in place.

camera lucidaCAMERA LUCIDA:
There is a Wollaston-type camera Lucida, which can be used for drawing, when the microscope is placed in a horizontal position. It can be slipped onto any of the eyepieces in lieu of their 'top-hat' caps.

OBJECTIVES:
Unusually, all the 20 achromatic objectives as listed in the 1875 P & L trade catalogue were originally included in this set, though the half inch 40o, is a period replacement by another maker. All have the RMS thread, first suggested in 1859. As several versions of each focal length were produced, each with different angular apertures, these are engraved here on the barrel of each objective, as well as on the cap of each objective can, in addition to their focal length. The practice of labeling the cans with the angular aperture of each objective is not commonly, or perhaps ever, found on other P & L examples. The objectives are all beautifully finished in resplendent lacquered brass, and range from 4 inches to 1/50 inch in focal length. Those having focal lengths of ½ inch and less have coverslip correction collars. The 1/25, and 1/50 inch are of exceptionally short focus, and were very expensive (resp. £21 0s, and £31 10s each). The 1/50 th is numbered “No 28”. The first example of this objective, now in the R.M.S. collection, was made for Dr Lionel Beale, and is dated Oct. 15th, 1864. Very few of these 1/50th objectives were made. One can see from the short focal length and the angular aperture, (not better than the 1/16th), that the 1/50 objective was an impractical lens which did not improve upon the resolution of lower power objectives, yet was much more costly. Water-immersion fronts are provided for the ¼-inch 160°, 1/8-inch 160°, 1/12-inch 145°, and 1/16-inch 175° objectives. These are in smaller marked cans, and are engraved individually with, e.g. “1/4-inch immersion”, or “1/16-inch wet”, and their dry counterparts accordingly. In the 1875 price list, these cost £2 2s extra each. There is also a Brookes-type double-arm angled objective changer(double nose-piece), which was added recently though the box had a cutout recess designed for it already.

ACHROMATIC OBJECTIVES FOUND WITH THIS MICROSCOPE:

FOCAL LENGTH (in.) ANGULAR APERTURE
(DEGREES)
N.A.PRICE, £, s (1875)
49 0.081, 10
3 12 0.102, 15
2 14 0.12 2, 15
1 1/2 20 0.17 3, 0
1 30 0.26 3, 3
2/3 32 0.28 3, 10
1/2 40 0.34 4, 4
1/2 70 0.57 5, 0
4/10 80 0.64 5, 5
1/4 95 0.74 5, 5
1/4 130 0.90 7, 7
1/4 140 0.94 9, 9
same with immersion front 1.26 2, 2
1/5 100 0.77 6, 6
1/8 100 0.77 7, 7
1/8 140 0.94 8, 8
1/8 160 0.98 9, 9
same with immersion front - 1.26 2, 2(front only)
1/12 145 0.95 12, 12
same with immersion front 2, 2 (front only)
1/16 175 0.99 16, 16
same with immersion front - 1.21 2, 2 (front only)
1/25 160 0.98 21, 0
1/50 150 0.97 31, 0

objectives with immersion caps

objectives objectives objectives objectives objectives

LEIBERKUHN REFLECTORS:
LieberkuhnsLieberkuhns, fitting over the objectives,are included to fit objectives ranging from 2 inch to 1/4 inch focal length. They reflect and concentrate light coming from below an opaque subject onto to the subject, as when using the Dark Wells, or when viewing small opaque objects.

DARK WELLS:
DarkwellsFitting in the substage sleeve is a holder for dark wells, of which 3 of the original 4 are still present. They serve to provide a dark surrounding for an opaque subject illuminated from above. This reduces glare and reflection near the subject, making it easier to see details.

CONDENSERS:
CondenserThe set includes Powell & Lealand’s well known Improved High Power Achromatic Condenser, with a wheel of 11 apertures, and 4 stops, operated by separate arms and pointers, recording positions from 0 to 4 on a dial. The different size apertures can be used, when the 0 position is selected, whereas it is advisable to select the largest aperture No 11, for use with stops 1 to 4, which produce dark ground and oblique illumination. The condenser can be used for high power with its top element on, or with the top element removed, as a moderate power condenser for objectives with focal lengths as long as one inch. This example still comes with its original top pinhole cap, used for centering purposes. With the top element included, this condenser has a reported angular aperture of 170o, and an n.a. of 0.99. It was not designed for use with oil immersion objectives, which did not exist when it was first designed.

Two additional P & L condensers were addded to this set by the present owner. These are shown in the next two paragraphs, and include the 'chromatic oil immersion' model and the 'achromatic oil immersion' model.


achr cond With the development of their first oil immersion objectives, Powell & Lealand’s dry achromatic condenser, with a maximum N.A. of 0.99 no longer provided a sufficiently large cone of light. To take full advantage of the higher resolving power, they first produced a chromatic (i.e. not achromatic and on the principle of a small Abbe) oil immersion condenser in 1880, which was principally designed to be used for providing oblique illumination. This had a total aperture (N.A.) of 1.30. Its operation is quite ingenious, and consists of a rotating sleeve , with at its top end both a fixed- and a moving diaphragm, the combination of which results in 4 different configurations: one central aperture for focusing/centring purposes, two oblique illumination pencils with one at 0 degrees, and the other at 90 degrees respectively, and a fourth combining the latter two, i.e. at right angles of each other. The optical element has a focal length of 1/5 inch, and in this particular example is ‘truncated’, i.e. all optical elements are combined in a single brass cell, which screws into the top of the sleeve. In the original form rotation of the sleeve to select the different apertures was achieved by operating a lever attached to its lower end. In this example this is achieved by moving a screw protruding from the inferior surface of the condenser below the substage, which runs in a semi-circular groove.


achr condWith the appearance of Powell & Lealand’s achromatic oil immersion objectives with the very great apertures of 1.4 and 1.5, and their introduction of apochromatic objective systems, their chromatic oil immersion condenser was subsequently succeeded by an achromatic condenser on the same immersion principle, but out of necessity mechanically different, so it could be employed for transmitted- as well as oblique illumination, and allow for the use of different removable stops. To work most effectively, stops need to be placed close to the lower focal point of the condensers’ optical parts, and inevitably, in high power condensers this is high up in the condenser tube near its lower lens element. As a result, the user was often forced to lower the condenser, thus bringing it out of focus, in order to change the diaphragm stops, when a different mode or angle of illumination was required, a significant disadvantage. To get around this problem, Powell & Lealand (at the instigation of Dallinger, see: Carpenter and Dallinger, 7th ed (1891), page 254)devised a rising and falling sleeve which carried the swing-out holder for the diaphragm stops, which acted as an ‘elevator’, taking the stops up to the required position, so that they could be changed whilst the condenser remained in focus on the slide. On this design, an achromatic version (N.A. 1.30) was constructed in 1886, and one on the apochromatic principle (N.A. 1.40) in 1891. Apart from being listed in Powell & Lealand’s own catalogues, this condenser was also advertised by W. Watson & Sons in 1902: ‘Powell & Lealand’s Apochromatic oil immersion condenser, 1.4 N.A. mounted in P & L’s special carries, with stops, complete £ 12 0 0”. Unfortunately, in this particular example of this rare device the front lens element is missing, and the range of stops is incomplete.


wheel of aperturesThere is a diaphragm with 4 rotating aperture stops and a blue filter, which inserts into the substage sleeve. In the catalogue this is referred to as Rainey’s light modifier.


Parabolic illuminatorspot condenserFor dark ground use, there is a Wenham-Shadbolt parabolic illuminator with adjustable center stop(left). There is also spot condenser for lower power dark ground work(right).

pole accessoriesPOLARIZED LIGHT ACCESSORIES:
These include a substage polarizer(A), nosepiece analyzer(B), and an attachable substage set of swing-out selenites(C). .

erecting lensERECTING LENS:
As noted above, this device is designed to screw in to the bottom of a drawtube. It corrects the inverted image that results from the use of an eyepiece and objective lens. This becomes important mainly when trying to perform a dissection under a compound microscope.

SIDE REFLECTORSILVERED SIDE REFLECTOR:
For illuminating opaque objects from the side, there is a silvered side-reflector on a pivoting arm, which inserts into a hole at the top of the square body pillar. This slot is drilled through to the other side, so the device may be used from either side, or one of the other accessories may be inserted.

STAGE BULLSEYESTAGE BULLSEYE CONDENSER:
Another accessory which fits into the hole on either side of the top of the square body pillar is a stage bulls-eye condenser on a pivoting arm. This is used to concentrate light onto the top of a specimen. This is also used primarily with opaque objects.

p and l frogplateFROG PLATE:
A large lacquered brass frog plate, measuring 170 x 45 mm, with a circular glass window accompanies this instrument. This would have been used to demonstrate the circulation of blood in the capillaries of the frog's foot web or the tail of a fish.

p and l compressorCOMPRESSORIUM:
A nicely finished and solidly made lever compressorium of generous proportions is included, onto which can be affixed a standard 3 x 1-inch object slide, for which there is a spring fitting in the base. The use of a standard slide for the bottom working surface is both practical and unusual.

p and l liveboxesLIVEBOXES:
There are two brass live boxes, one of which shows more extensive use, as per the loss of lacquer on either side of the chamber.

maltwood finderstage micrometersSPECIAL SLIDES:
A Maltwood’s finder in Morocco leather case, as well as a stage micrometer, both items included in the 1875 Powell & Lealand catalogue, have been added to the kit, and though contemporary, are not original.

bench bullseye condenserBULLSEYE BENCH CONDENSER:
Accompanying the instrument is a large and heavy bullseye condenser on a stand. This allows the flexibility of directing concentrated light both above and below the stage and adjusting the lens without disturbing the microscope itself.

pl prismpl prismACHROMATIC PRISM:
A rectangular achromatic prism for oblique illumination, fits into the stand of the bench bulls-eye condenser.



HISTORY OF THE POWELL & LEALAND No 1 MODEL MICROSCOPE STAND

1843 p and l standAlthough larger model microscope stands had been produced by Hugh Powell, notably the massive stand commissioned by the Royal Microscopical Society in 1840, what is now known as the No 1 Model was a direct development of Powell & Lealand’s new tripod-based stand of 1843, seen to the left, click on the image for larger and additional images. As shown here, this stand was first described in that year’s November issue of The London Physiological Journal as “Powell & Lealand’s new microscope”. This is supported by a lighter version of the tripod, which carries the body on trunnions, and a transverse arm, which contains the long lever, which moves the nose piece fine-focus adjustment. Initially, the fine-focus screw, which operated a cone, was placed on the right side of the arm. In 1847 this was moved to the familiar vertical position on the top of the arm, immediately behind its pivot. At this stage, the microscope tube was also supported by a set of diagonal stabilizing struts extending from the back of the arm to the upper third of the tube. Sometime during the early 1860's these were abandoned.


p and l 1856 Powell & Lealand did not enter the 1851 Great Exhibition, but it is believed that during this time they worked on a larger model based on this design. Dated examples are extant from this period, with the more solid rectangular feet as in the later No 2 stand, either with or without the supporting struts. These could be regarded as the 'proto-No 1/No 2 stands', from which the later models were developed. An example of one of these, called the Improved Large First Class Microscope is found on this site, and still has struts supporting the tube. As seen in the image to the left, this stand of 1856, is similar to the 1843, stand other than the position of the fine focus knob, the rack and pinion substage, the design of a heftier tripod, and the rectangular and heavier tripod legs.


No 1 about 1861 The next model(left) was worked on at odd times, and appeared in 1861. It was similar in outline and appearance to the final No 1 design, save for the massive ring attached to the limb, within which there was a second ring carrying the stage to which the substage was attached, allowing both to be rotated by the same rack-and-pinion movement, with separate rotation also provided for the substage. An example of this model was ordered by the Radcliffe Library for use by The University Museum, Oxford, in 1864, together with a 1/50-inch objective amongst the accessories. This model continued to be made until the definitive version of the No 1 model stand was put on the market in 1869. The main modifications involved the construction of the rotating stage, which this time was mounted in the same plane as the ring, and there was a separate carrier for the centering and rotating substage, attached below the stage now, and not rotating with it as in the previous model. In this form, the Powell & Lealand No 1 microscope continued to be made unchanged for another 40 years, until the firm quietly faded out of existence during the first decade of the 20th Century. The only additions were a fine adjustment to the substage (1882), rack work to the monocular draw tube (1887), and diagonal rack work to the coarse adjustment, all at the suggestion of E.M. Nelson, an eminent microscopist, and user of this model. After the appearance of Zeiss apochromatic objectives in the late 1880s, an exchangeable shorter continental drawtube of 160 mm was also made available, this being one of the few microscopes at the time with a fine adjustment precise enough to take full advantage of these. Not long after this, Powell & Lealand developed their own apochromatic objectives for the longer 250 mm English tube.

Dallinger with his and the present microscopedallinger The Reverend Dr. William Henry Dallinger, F.R.S. (1839-1909), parson-naturalist and microscopist, was an enthusiastic user of the Powell & Lealand No 1 microscope, and the 1891 and 1901 editions of Carpenter’s The Microscope and its Revelations, edited by him, include four engraved plates depicting the No 1 stand in use for different methods of observation. The painting of Dallinger in the Wellcome collection by Edgar Herbert Thomas, shown to the left, depicts him in academic gown seated at a binocular Powell & Lealand No 1 microscope. Note the similarity to the microscope pictured and discussed on this page. Dallinger also used a special stand, allowing the microscope to be used vertically for fluid mounts(right). He conducted research into saprophytic organisms, and with the extensive signs of use on the fine adjustment, and on one of the live boxes, it is tempting to speculate that he may have been the original owner of this very microscope, particularly considering the striking resemblance to the stand here discussed. Dallinger, being an author of books about the microscope, would be a good candidate to want to have all the various P & L objectives to write about in his book. Definitive proof to support this hypothesis is lacking, however.



HISTORY OF THE POWELL & LEALAND FIRM

Hugh Powell(left) was born in 1799, and in his early working life he was a philosophical instrument maker to the trade, which in those days meant that one’s work was often signed and sold by other retailers. The first mention of him is in 1831, regarding the first of three simple dissection microscopes made by him for Cornelius Varley (1781-1873). They both lived in the same neighborhood of London, Somers town. Varley was an inventor himself, and patented his graphic telescope in 1811. He was also a microscopist, and a painter, his favourite medium being watercolours. He was also one of the 17 gentlemen who met at the house of Edwin Quekett in 1839, to form the Microscopical Society of London, later the Royal Microscopical Society. It is also known that Powell was making mechanical microscope stages designed by the engraver, Edmund Turrell, who also produced some of Varley’s illustrations for The Transactions of The Society of Arts, so there were many interconnections between men involved in microscopy.

Varley’s nephew was Andrew Pritchard (1804-1882), and for some time Hugh Powell made a number of microscopes for him, whilst other examples of this Pritchard design were signed and retailed by other opticians, such as Bate, and Dollond. It would appear that Hugh Powell started signing some of his own instruments in 1840, and a few examples exist which still feature his early fine adjustment, which worked through a wedge under the stage, and this less-satisfactory arrangement was later abandoned.

powell microscopepowell microscope In 1840, the newly formed Microscopical Society commissioned Messrs Hugh Powell, Andrew Ross, and James Smith each to furnish a standard instrument made to their own peculiar views. Powell’s massive model(left) as delivered to the Society is still part of the R.M.S. collection today, and features a solid triangular post, as well as the first example of an English microscope equipped of an achromatic condenser. A small number of this large model continued to be made until 1849.

Sometime in late 1841 Powell took into partnership his brother-in-law Peter Lealand, thus forming the famous firm of Powell & Lealand. Their first joint venture was to bring out a different large microscope based on the Jackson optical-bench principle, which featured a conical fine-adjustment mechanism, which carried the weight of the upper body and tube, and as such was prone to wear. This model, shown to the right, was illustrated in Daniel Cooper’s: “The Microscopic Journal and Structural Record” for 1841, and later published as a separate brochure by Messrs Powell & Lealand in 1842. Descriptions of the microscope are also found in other sources. In a later version of the instrument, the fine-focus arrangement has reverted to nose-piece adjustment. It would appear that at this stage Powell & Lealand were also still manufacturing for the trade, as examples of their work are known signed and retailed by others, including Carpenter & Westley, and Dollond.

1843 p and l stand In 1843 Powell & Lealand came out with a radically new design on the bar-limb principle, combined with a tripod foot. An example of this 'New Microscope' from the first year of its production is now on this site. This coincides with the appearance of Andrew Ross’s first bar-limb design microscope, but the tripod foot, which resembled a kettle-drum, was a distinct Powell & Lealand feature. This was described in that year’s November issue of The London Physiological Journal as “Powell & Lealand’s new microscope”. In this, the microscope is supported by tripod legs, and small rounded feet, which carry the body on trunnions, and there is a transverse arm which contains the long lever, which moves the nose-piece fine motion. In the initial design, the fine-focus screw, which was attached to a cone, was placed on the right side. In 1847 this was moved to the familiar vertical position on the top of the arm, immediately behind its pivot.

portable Coinciding with this development, a portable version of this model appeared, with folding feet, allowing the instrument to be packed neatly in a compact box. This was later known as the 'portable' or No 4 model(right), and the original design became known as the No 3 model. At this stage, the microscope tube was also supported by a set of diagonal stabilizing struts extending from the back of the arm to the upper third of the tube. Sometime during the early-1860s these were abandoned.


iron In 1848, a student model was developed, later referred to as 'The Iron Microscope' (left). This had an off-set pillar and flat tripod stand, a single coarse-adjustment milled head placed on the right, and a lever-operated stage as designed by Alfred White. Some examples of this instrument had a foot wholly made in brass, and some were signed by other retailers, such as Carpenter & Westley. This model continued to be made until the first decade of the 20th Century, and was later known as their No 5 or Student Model.


p and l 1856 Powell & Lealand did not enter the 1851 Great Exhibition, but it is believed that during this time they worked on a larger model based on their 1843 tripod design. Dated examples are extant from this period, including the one on this site, which have the more solid rectangular legs and feet as in the later No 1 and 2 designs, either with or without the supporting struts. These could be regarded as the “proto-No 1/No 2 stands”, from which the later models were developed.

No 1 about 1861 The next model(right) was worked on at odd times and appeared in 1861. It was similar in outline and appearance as the final No 1 design, save for the massive ring attached to the limb, within which there was a second ring carrying the stage and substage assembly, allowing both to be rotated with the same rack-and-pinion movement, and separate rotation also provided for the substage. This model continued to be made until the definitive version of the No 1 model stand was put on the market in 1869.

The main modifications made from 1869 on were in the construction of the rotating stage, which this time was mounted in the same plane as the ring, and there was a separate carrier for the centering and rotating substage, attached below the stage now, and not rotating with it as in the previous model. In this form, the Powell & Lealand No 1 microscope continued to be made largely unchanged for another 40 years. The only additions were a fine adjustment to the substage (1882), rack work to the monocular draw tube (1887), and diagonal rack work to the coarse adjustment, all at the suggestion of E.M. Nelson, an eminent microscopist, and loyal user of this model.

no 2 Alongside the No 1 model stand, Powell & Lealand developed another, slightly smaller model, known as their No 2 (left). This differed from their No 1 in four respects: Instead of 1 inch, it had ¾ inch X and Y motion to the stage; instead of a rotating stage, it had a sliding and rotating object holder; the stage plates were not graduated; and when inclined horizontally, the eye point was a distance of less than 10 inches from the table surface. It did feature the same centering and rotating substage as on the No 1.

no 3In addition, Powell & Lealand continued to produce their original design with the ranker tripod foot, now designated their No 3 stand(right). This differed from the No 2 in that it was smaller again, had no rotary movement to the substage, and instead of rectangular mechanical movements to the substage it had mechanical eccentric centering movements.

As a firm, Powell & Lealand were always small, and remained a family business, with Hugh Powell’s son, Thomas (1833-1925), being an active member of the team, and taking over after his death in 1883. Hugh Powell alone worked at 24 Clarendon Street, Somers Town, London from 1832 to 1842 when Peter Lealand joined him. They subsequently moved to Seymour Place, Euston Square, London, where they were located from 1846-1857, and from 170 Euston Road, London (same premises), from 1857-1905. In later years, Thomas was almost solely involved in the production of the microscope objectives, and it was reported that he worked in great secrecy, always covering his work before the entrance of visitors. Dr Henri Van Heurck, in his well-known text “The Microscope”, says of the firm: “Messrs. Powell & Lealand occupy quite a unique position in the microscopic world. Their workshops are small, the number of instruments which they produce are few, but every piece of apparatus, marked with their name, is an artistic production, perfect in all its details. Moreover, both instruments and objectives of these makers are in the greatest request, and are used in England by all serious microscopists”.

Towards the end of the 19th Century, competition from German firms, such as Zeiss and Leitz, with their mass-production techniques, and apochromatic objectives, which could be produced less expensively, caused demand for Powell & Lealand’s products, which largely catered for the wealthy amateur, to fall. In the end, there were only a few staff left, including Thomas Powell, and Mr Lealand junior, alongside foreman Charles Perry. After Powell & Lealand ceased making for themselves, a number of stands were sold by Swift, under supervision from Charles Perry. He later moved to work for C. Baker in 1904, and made No 1 stands to be approved by Thomas Powell until 1914. At that stage, the address of the firm was given as Elmstdale, 87 Greenham Road, Muswell Hill, London. Thomas Powell continued to service and repair his instruments almost to his death, in 1925.

HISTORY OF POWELL & LEALAND OBJECTIVES:

Hugh Powell, whilst working for the trade, must have started making microscope objectives in the mid-1830s. He was a pioneer in the production of very high powers in objective lens systems, and made a 1/16-inch objective as early as 1840. His son Thomas followed this tradition, by making a 1/25-inch dry objective in 1860, and a 1/50-inch in 1864, for Dr Lionel Beale. Dr Beale persuaded him to make the even more remarkable 1/80 objective in 1872, which Beale mentions in his 5th Edition of “How to work with the microscope” (1880). As far as is known, this is the only example in existence.

It must be mentioned that at this stage there was still no knowledge of Abbe’s theory of diffraction, and the associated concept of numerical aperture (N.A.), a measure of the resolving power of objectives. This dictates that as a measure of resolution:

       N.A. = sin α x R.I. (refractive index of the medium)

       α being half the angular aperture. For air, the R.I. is 1.0, for water 1.33, and for modern immersion oil 1.515.

The early microscopists, and especially the élite of rich English amateurs, who amused themselves by resolving the fine details of test objects such as diatoms (scathingly known as “diatom-dotters”), strived to acquire objectives with greater and greater angular aperture, at that stage not realising they were really chasing an asymptote. For instance, the gain in resolving power of a 175° over a 160° objective is negligible, considering their respective N.A.s are 0.98 and 0.999!

The immersion principle had been experimented with earlier by David Brewster, and the Italian professor Giovanni Battista Amici, who demonstrated a water-immersion objective at the Paris Exhibition in 1855. At first, it was not realised that immersion would increase the effective aperture, and the prevailing thought at that time was that it only tended to reduce the loss of light through the air-glass interface. This theory was also supported in England, especially by authorities such as Francis Wenham. However, on the Continent, more interest was shown by opticians, including Edmund Hartnack (1826-1891), and Friedrich Adolf Nobert (1806-1881). Hartnack impressed the judges at the London Exhibition of 1862 with his water-immersion objectives, which were better than Continental dry objectives in resolution. It is thought that the superiority of Powell & Lealand’s dry objectives, such as their 1/25 inch, which were about equal in resolution as Hartnack’s immersion, caused it to take longer for the score to be settled in favour of immersion objectives, such as was finally demonstrated by the microscopist John Mayall (1842-1891).

Spurred by this new evidence, Powell & Lealand produced their first water immersion in 1869, an 1/16-inch objective. Water immersion was at this stage available as an optional extra for their high-power objectives, in the form of “immersion fronts”, which replace the “dry” front lens, simply by unscrewing and replacing one with the other. The 1871 catalogue lists this arrangement for the 1/8, 1/12, and 1/16-inch objectives. The 1875 catalogue adds this feature also for the ¼-inch 140° objective.

In the USA, Robert Tolles had experimented initially with water immersion, but in about 1873 he produced a 1/10-inch objective using soft Canada balsam as an immersion medium, which had an aperture of 110°, equivalent to an N.A. of 1.25, but was less than practical, due to the viscosity of the balsam. Around this time, Ernst Abbe followed up on the idea of homogenous immersion generated by Stephenson, and published his famous paper in the Journal of the Royal Microscopical Society in 1879. Instead of water, a drop of oil with the same refractive index of glass (about 1.52) was placed between the front lens of the objective and the cover glass, taking the theoretically possible maximum N.A. up to this number. Not long after, Powell & Lealand produced their first oil-immersion objectives, commencing with a 1/12 inch with an N.A. of 1.20, followed by a 1/25 inch of 1.38, and even a 1/50 inch with the same resolution. A 1/12 inch with a hyper-hemispherical front lens with an N.A. of 1.43 followed in 1880, and 5 years later, they came out with their oil immersions of 1/8, 1/12, and 1/50 inch, all with flint-glass front elements of a higher refractive index, producing a remarkable N.A. of 1.50, close to the theoretical maximum. Despite the high N.A., these lenses were not very practical, as they were less well-corrected, suffered from very short working distances, and especially the 1/50 inch produced only “empty magnification”, which revealed no additional detail.

A further development in microscope optical design was the appearance of apochromatic objectives in the early 1880s through the work of Ernst Abbe of Zeiss, using the new Schott optical glass, which had lower dispersion properties, and allowed correction for three (blue, green, and red) colours instead of two (blue and red), as in the achromatic lens combinations. As they were over-corrected for some colours, they were to be used with compensating eyepieces, which counteracted this. In 1882, an addition to the substage, was the substage fine focus mechanism to facilitate 'Critical Illumination' at high power.

About 1892, Powell & Lealand started manufacturing their own apochromatic objectives, using Jena glass, and made some remarkable objectives including oil immersions of 1/8, 1/10, 1/12, 1/20-inch focal lengths, with N.A.s of 1.40, and even 1/10, 1/12, and 1/20-inch objectives, each with an N.A. of 1.50. Naturally, these were expensive, and ranged in price from £20 to £50. Unfortunately, the earlier Schott glass proved unstable, causing de-vitrification, rendering the lenses opaque, and today few, if any, of these objectives are still in useable condition.

Towards the end of the 19th Century, the small firm of Powell & Lealand, even in England, had started to suffer from the competition from the German makers of apochromatic objectives, and their catalogues of 1896 and 1899 suggest they lowered their prices substantially. They even advertised a new apochromatic 1/12-inch objective with an N.A. of 1.40 for only £10, with the addition: “This objective is a rapid photographer”. Alongside this, they made a dry and an oil-immersion apochromatic condenser. After they ceased trading in 1901, for some time their objectives continued to be advertised by other makers, such as W. Watson & Sons in their 1902 catalogue, before this firm started making their own apochromatic lenses.

HOW DOES THIS MICROSCOPE PERFORM WHEN USED TODAY?

The first observation to make is that this is an instrument for an experienced microscopist of the “old school”, as unlike in modern research microscopes, nothing is automatically set, and everything needs to be attended to manually, and a good understanding of microscope optics, lighting, etc, is a prerequisite.

It is a well-proportioned instrument, with a beautiful finish, and the ergonomics are superb, allowing for a comfortable eye and head position when placed at the correct angle for the individual user. All the controls are smooth and precise, with no slack, and friction can be altered by adjusting, e.g. the screws, for the coarse focus with a thin slotted screwdriver.

Users accustomed to older microscopes fitted with larger-diameter Abbe-type condensers immediately experience the peculiarities of a shorter-focus, smaller-diameter condenser, which requires more careful centering, and this is even more delicate, when the mirror is used. One finds oneself taking the eyepiece out several times, to ensure the light-source diaphragm, and one of the selected aperture stops of the achromatic condenser, all line up. When new, this microscope would have been used with an oil lamp, i.e. a small light source, whereby the edge of the wick was projected in the image plane as per E.M. Nelson’s method, and modern electric light sources tend to produce a significant amount of glare, which can only be overcome by shutting the lamp aperture down to just coincide with the size of the image field. The high N.A. of the condenser also necessitates a smaller condenser stop to be chosen (e.g. No 5 or 6 out of the total 11), even with the higher-power objectives, to prevent flooding the image with light.

The objectives are things of beauty, but especially the very high powers, and those with a high angular aperture are tricky in use, due to their very short working distance, and the constant need to use the cover-thickness correction collars. The best method is to bring the fine adjustment slightly out of focus, so that a small dot in the field appears as a dark ring just above or below the focal point, and then to refocus using the correction collar. The procedure needs to be repeated several times for the best result. Otherwise, when no further correction is possible, one can shorten the draw tube.

Most modern slides have a cover glass thickness of about 0.17 mm, and some slides cannot be focused with the 1/16, 1/25, and certainly the 1/50-inch objectives, even when the collar is taken to its maximum correction. Lionel Beale reports in his 1880 edition of “Working with the Microscope”, that the cover glass suitable for use with the 1/50-inch dry objective had to be 1/20th of a millimetre thick, or 0.05 mm, and Powell & Lealand had this specially made for them by Messrs. Chance, of Birmingham. I have found that most diatom test slides, such as those labelled by Watson, suitable for use with the 1/25 and 1/50 objectives, can be focussed with these, but inevitably, empty magnification is obtained. It is commonly accepted now that no new detail is revealed when the image is magnified more than 1000 x the N.A. of the objective. Based on this, E.M. Nelson introduced the concept of “Optic Index” (O.I.), being the magnification of the most powerful eyepiece which could be used with an objective before empty magnification sets in. For the dry objectives in this set, the 1/8-inch 160° objective has an O.I. of 12, whereas for the 1/12, 1/16, 1/25, and 1/50 -inch objectives this number is 8.2, 6.1, 4, and 2, respectively.

It is therefore not surprising that the most useable objectives in the set are those of smaller angular aperture. When the highest degree of resolution is required, the 1/8-inch 160° objective, both “dry” and “wet”, gives the finest resolution. Even when used dry, it just resolves the striations of Ampipleura lindheimerii in oblique light.

The Wenham binocular tube works best with the low-power stereoscopic prism, with objectives up to ½ inch, with the x 5 and x 7.5 eyepieces. The x 10 Kellner eyepieces sit deeper in the tube, and require the use of spacers to get the eye points far enough apart. The high-power prism works reasonably well, but the image for the left eye is a lot fainter, and with higher magnifications this becomes an issue.

The wide range of accessories renders this a very versatile instrument, and it is suitable for bright and darkfield, reflected, and polarized light examination of specimens. It is a relic of the bygone era of handmade instruments produced by the last of the artisan instrument makers, the true “Rolls-Royce Silver Ghost” of all microscopes.

BIBLIOGRAPHY

Beale, Dr Lionel. How to Work with the Microscope. 5th Edition, 1880

Bracegirdle, Brian. Notes on Modern Microscope Manufacturers. Quekett Microscopical Club, 1996

Bradbury, S. The Evolution of the Microscope. Pergamon Press, 1967

Carpenter and Dallinger. The Microscope and its Revelations. 7th (1891), and 8th (1901) Editions

Disney, AN. Origin and Development of the Microscope. Royal Microscopical Society, 1928

LaRue, B.J. Powell and Lealand's New Microscope of 1860. Microscopy. pp139-142. 1988.

LaRue, B.J. The Powell and Lealand Number Two Microscope. Journal of the Scientific Instrument Society (England)pp7-8 1989.

LaRue, B.J. The Powell and Lealand Number Three Microscope. Microscopy. pp444-455 1990

Malies, H. A Short History of the English Microscope – The 19th Century Instrument. Microscope Publications, Chicago, 1981

Nelson, EM. Hugh Powell’s Microscopes. Journal of the Royal Microscopical Society, Pp 282-97, 1900

Nuttall, RH. Microscopes from the Frank Collection 1800-1860. Fig. 30, P 42. Frank, 1979

Quekett, John. A Practical Treatise on the use of the Microscope. Baillière, London, 1848

Turner, GL’E. Hugh Powell, James Smith, and Andrew Ross: Makers of Microscopes. In: Mid-Nineteenth Century Scientists. Ed John North, Pergamon Press, 1969

Turner, GL’E. Powell & Lealand, trademark of perfection. Essays on the History of the Microscope. Senecio Publishing Co., 1980

Turner, GL’E. Collecting Microscopes. Littlehampton Book Services Ltd, 1981

Turner, GL’E. The Great Age of the Microscope. The Collection of the Royal Microscopical Society through 150 Years. Taylor & Francis Ltd, 1989

Van Heurck, Henri. The Microscope. English Edition. Crosby Lockwood and Son, London, 1893

White, Alfred. On a Lever Movement to the Stage of the Microscope. Transactions of the Microscopical Society of London Vol I, 1844. pp165-166, and plate 20.