THE VARLEY 'CAPILLARY TABLET'

BROOKE TYPE:

This type of compressor is one of a class known as 'gravity compressors.'   This means that the pressure applied to the specimen is limited to what the weight of the top pressure plate applies without any leverage or springs to apply a greater amount of force. The bottom plate of brass has a central hole. Over this hole is the bottom support for the specimen made of glass. The top plate, with thin glass on its bottom surface, rides freely on two vertical pins. The distance of the bottom of this top plate is regulated by three thumbscrews which project from its bottom and act like little 'legs.'   As each screw is loosened, the weight of the top plate brings the glass on its bottom closer to the glass on the bottom plate. The maximum pressure applied, is simply a function of the weight of the top brass plate. This device has advantages of easy access, low cost, and easy cleaning. Disadvantages include the need to adjust three different thumbscrews to apply equal pressure all around the object being studied, and the fact that more severe pressure, beyond the weight of the top plate, cannot be applied.

LISTER TYPE:

The Lister compressor, also referred to (by Beck) as a 'screw live box,' was first manufactured by James Smith. It is a compressor with springs to apply greater pressure than that of the gravity compressors. It consists of a bottom brass plate, with a cylinder permanently erected upon it; at the top of this cylinder is permanently mounted the bottom glass surface, just like a live-box. The difference though is that the outside of this cylinder is threaded and a large knurled ring rides in these threads. The top plate of the compressor rides on two upright posts, with springs applying downward pressure against the knurled ring on the bottom plate cylinder. If the knurled ring is screwed all the way down, the glass in the top plate is pressed against the glass at the top of the cylinder with the full force of the springs. If the knurled ring is unscrewed, it pushes up on the top plate, lifting it off the bottom glass.

The glass on the bottom part has beveled edges. There are two steel tabs on top of the top plate to hold a cover slip in place. When the ring is unscrewed, it pushes up on the top plate, and when screwed down, it allows the top plate, pushed down by the springs, to compress the specimen upon the bottom glass support. In effect this allows a fine degree of adjustment to the degree of compression, which is more forceful than the gravity types due to the springs.

In use, the knurled ring is unscrewed all the way up with no cover slip on the top plate. The specimen is then placed on the glass at the top of the cylinder of the bottom plate; once the specimen is in place, a cover slip is placed under the two steel clips of the top plate, and the knurled ring is slowly screwed down until the desired degree of compression is achieved. The top plate has a straight edge against which a coverslip with one flat edge can register.

The Lister compressor was made and sold by James Smith no later than 1840, continued to be sold by his successors and was still offered in the 1891 catalog of James Swift & Son and the 1894 catalog of Smith's successors R. & J Beck. Ross also made a variation of the Lister compressorium, with four instead of two upright posts. The Lister compressor has an advantage over many others in that it is a very simple matter to change the top cover glass by simply moving the spring steel fingers away.

YEATES TYPE:

The Yeates compressor, first described in 1842 by George Yeates of Dublin, has a bayonett fitting on the bottom to fit in the center of a suitable stage. Such a stage would be part of some examples of Pritchard microscopes. It had a removable center holding the bottom plate of glass and a design that kept the two glass plates parallel. With the center supporting tube removed, the stage clips on top could support an ordinary slide. At the time this device was invented it was not appreciated that this distance from the stage opening was a disadvantage reducing the ability to use higher powers to greatest advantage.

LEVER or SCHIECK TYPE:

ROSS SWING-OUT COMPRESSORIUM STAGE (1862-72):

This compressor was apparently first exhibited by Thomas Ross in the Great Exhibition in London in 1862. It was designed to sit firmly on top of the existing stage with pins holding it in place. The compressorium could be completely swung out of the way, leaving the stage with a typical slide rest. In operation, a normal slide and cover slip would be used, and the compressing plate, N-M would be lowered on top of the cover slip with pressure regulated by the knurled knob P. (Thanks to Jim Solliday for information and images of this compressor).

PARALLEL PLATE TYPE (BECK):

The Beck Parallel Plate compressor is constructed in such a way that it resembles a parallelogram. Its chief advantages are its thin profile and reversibility. The illustration above to the right shows one of the glass supports removed. There is one for the top plate and one for the bottom. Cover glasses are glued to these metal disks which lock into place and it makes it a much simpler matter to clean them or replace them. Again, a glue that contains some beeswax would make removal easier. Most surviving examples of the Beck Parallel plate compressor are missing these round metal fittings to hold the cover glasses. Early versions did not use these fittings, but simply used small knobs to fasten large rectangular coverslips to the plates and an example of this type is on the site on another page. To use this device, the two plates are pushed together after the specimen is placed on the bottom glass. The springs push the two plates apart thus maintaining tension. When the two plates are relatively close to one another, as the horizontal screw fixed to the lower plate is advanced (A-->B), its tip engages with an angled channel(*) in the black metal attached to the top plate, drawing the top plate downward. Although it appears this device would maintain parallelism, as illustrated by the image below, this is not necessarily the case. In addition, as it is tightened further, it can bend.

ROSS' REVERSIBLE TYPE (?1869-?):

Ross made two versions of a reversible compressor. This is the thinner and simpler of the two. In his 1869 and 1872 catalogs, Ross offered a 'Ross Reversible Compressorium' By 1875, the 'Ross Improved Reversible Compressorium'(IRC) was listed. Strangely, the IRC was pictured in the 1875 catalog, but the illustration in the 1883 catalog seems to be the this less sophisticated model, with the caption still referring to the IRC. This model is harder to use than the next entry. In use, according to Dr. de Groot, the user needs to apply some manual pressure to the top plate while tightening the knob in order for it to properly compress the subject. The top plate rides in the angled grooves(*) on the right and left side of the bottom plate, and as the screw is tightened the top plate is pulled downward as it pushes further into the matching grooves. There are convenient tabs to hold cover glasses on each inside surface of the compressor. (Collection of Dr. Jurriaan de Groot).

ROSS' IMPROVED REVERSIBLE TYPE (1869-):

The 'Improved Ross Reversible compressor,' first reported in the Monthly Microscopical Journal of 1869, was apparently offered in Ross catalogs from 1869 through at least 1883. The main part of the instrument consists of an 'I-beam'-shaped base plate with a threaded hole into which screws a knurled ring onto which a glass base plate is glued. On the opposite side, as shown to the right, a secondary plate, A, also with a knurled theaded ring carrying a thin glass plate, floats between three keeper posts, B.   This secondary plate is pushed away from the main plate by two springs pushing up from the bottom plate(not shown, but virtually identical to the those in the preceding Ross entry). The secondary plate has wedge-shaped top edges, C.   The two-pronged fork, with a wedge-shaped tine on each side, D, sits on top of the floating secondary plate. As the worm screw, E, is turned, it advances the fork which pushes down on the secondary plate. The tines of the fork are kept in contact with the bottom plate by a ledge, F, on either side of the compressor. This compressor was apparently Ross' answer to the Beck Parallel Plate compressor as the catalogs referred to it 'for securing parallelism of the surfaces.'   To this author, this type is more likely to achieve that goal than the Beck, which could bend as tightened and did not remain parallel despite its parallelogram design. This model of the Ross Reversible Compressor seems to work much better than the other Ross Reversible Compressor. Again like several other examples, the cover glasses must be glued into the knurled rings; beeswax in the glue would make them easier to remove or replace when needed.

PIPER TYPE:

The Piper compressorium was reported in the JRMS in 1868 by Samuel Piper. This compressor has the advantage of complete rotation, allowing a variety of angles for viewing without removing the compressor from the stage. It is limited by the small size of the compressor itself, and the fact that the compressor is considerably elevated above the stage. The other end of this compressor is a cork-filled disk revolver. This would be used to observe an opaque object. The disk can be rotated by its milled edge, and/or turned at an angle from the horizontal. This is similar to the Beck Disk Revolver, except that rotation of this device is directly with the fingers rather than via a knob communicating with the disk by a fusee chain. The compressor accepts 22 mm diameter glass disks, held in place by the small knobs closing the rings around them. These are slightly thicker than coverslips, though coverslips might be able to be used. The large knob controls the distance of the two compression rings from each other. The cutout in the base is to allow the small cylinder of the compressor to clear the slide when swiveling away from the center without having to disturb the orientation of the disks. Another example on this website does not have the Swift signature,but does have the typical fancy Swift finish to the brass. (From another Private Collection). Piper also had several other contributions to microscopy.

WENHAM TYPE:

The Wenham type, with its thinner profile, was said to be especially suitable for use with dark ground illumination, as when using a Paraboloid condenser. The top plate is threaded to accept the knob. The top plate is springy and when the knob is turned counterclockwise, the top plate is lowered on the object being studied.

FOREST TYPE:

The Forest compressorium at first glance appears the same as the Wenham, but actually is the reverse, as tightening the knob moves the top plate down. In this case, the amount of pressure applied is not limited by the springiness of the top plate, since turning the knob applies the pressure and the springiness serves to keep the top plate off the subject when the knob is unscrewed.

GRAHAM TYPE (c. 1880):

HARDY TYPE:

In the early 1880's, Phin and also Hardy both described types of gravity compressor which was hinged on one end. This design was later modified and improved by Martin. The original description of this compressor is in both the JQMC and also the JRMS.

G. WATSON'S REVERSIBLE COMPRESSOR:

This compressor was noted in a brief note in the RMS Microscopical Journal, of 1886, page 520. Similar to the Piper instrument from 18 years earlier, its rotating parts are larger and therefore easier to handle, but a potential disadvantage is it requires a longer working distance to the objective when the subject holder is tilted. I would like to thank Dr Brian Stevenson for his kindness in supplying images of this compressor.

BULLOCH TYPE:

This compressorium, supplied with microscopes made by Walter Bulloch, is similar to the Wenham type on one side, but also incorporates features similar(but not identical), to the Hardy on the other- an added feature of a second knob on the opposite side of the specimen so as to apply pressure to the top plate from that side if needed or to increase the parallelism of the arrangement.

DELAGE REVERSIBLE TYPE:

This compressor, devised by Professor Y. Delage in 1886 is similar to a Schieck-type lever compressor, except that instead of using a lever, a screw acts on a wedge attached to the top plate holder. This type of compressor was made by Nachet. Like Schiek's original design, it has three pins standing up from its surface, allowing it to be reversed on the stage.

GRAVITY COMPRESSOR WITH DIAMOND-SHAPED TOP PLATE (c. 4th Qtr of 19th C.):

This is a later version of the gravity compressor in its simplest form, as the compression is not adjustable and depends solely on the weight of the top plate. This design is not rare and examples exist in various collections including that of the Royal Microscopical Society. This example is signed 'REG^D 244895.'   Although this number seems to date to 1870 in the British Registration database, it does not seem to correspond to this device.

ROUSELET TYPE (c. 1890's):

This compressor was invented by Charles Rousselet, FRMS 'about 1890' and was first noted in the JRMS in 1893. The general pattern of the top plate riding on a post 90 degrees from the bottom plate and the shape of the top compressor plate is quite similar to that of the Ross Swing-out Compressorium Stage of many years earlier. In 1903, Cross & Cole stated in Modern Microscopy that this compressor was the best type then obtainable. (Also, see Rousselet, CF, A Description of the Rousselet Compressorium, JQMC vol IX, 1904-1906 p 137-140). This original design is far superior to the later modified designs (an example follows this entry), promulgated by various makers.

ROWLAND'S REVERSIBLE COMPRESSOR (c. 1891)

Another example of the type of reversible compressor invented in 1868 by Piper but this time attributed to W. Rowland. It has a much simpler construction and utilized coverslips cemented to the loops instead of fitting into recesses in the loops.

WATSON'S IMPROVED ROUSELET COMPRESSOR (c. after 1895):

This is one of the modified Rouselet compressors, which makers like Watson called their Improved version. These modified versions made some things like changing the glass easier while at the same time making them less useful by introducing factors which detract from the original design as can be seen in the more detailed page concerning this example by Watson. Rousselet himself suggested that the proper method of gluing the coverslip to the movable top plate was to apply a thin layer of Marine glue, let it dry for thirty minutes, and then apply the coverslip; the slip was warmed causing it to adhere to the Marine glue; the warming could also be used to remove the slip if it was broken. Using glue instead of just pressure to hold the coverslip in place eliminated the risk that oil from an oil immersion lens would leak into the fluid holding the specimen.

HIGH PRESSURE COMPRESSOR FOR DETECTING TRICHINOSIS (after 1870):

This type of compressor came into widespread use about 1870 when Virchow convinced the German authorities that microscopical meat inspection was critically important in prevention of trichinosis. They continued to be used well into the twentieth century and large numbers of specialized and relatively inexpensive low power microscopes known as 'Trichinascopes' were produced especially in Germany. This example, from a model Vb trichinoscope, dates from the early 20th Century. In use a piece of meat (muscle) was teased out of the carcase with tweezers, placed between the two thick glass plates, and subjected to high pressure to make it relatively transparent. Low power was used to scan the meat and higher (medium) power was only needed to confirm what could be seen with low power. Today this process is different as the meat is digested with enzymes, allowing the larvae to sink to the bottom of the the solution and be more easily detected using a stereo microscope.

BARNARD TYPE (2nd Qtr 20th C.) The Barnard compressor was first described by J.E. Barnard and F.V. Welch in the December 1928 issue of the Journal of the R.M.S. It was offered in the R. & J Beck Catalog of about 1930. It was designed for use with a high power objective and illuminator. Joseph Edwin Barnard(1870-1949), was a highly regarded amateur microscopist, who served three terms as president of the R.M.S. and wrote the book Practical Photomicrography which persisted through three editions. He was also known for his improvements in the ultraviolet microscope and his work with viruses.

TAYLOR TYPE (4th Qtr 20th C.):

The Taylor compressor is a modern version of the Rousselet.   It features high quality construction and stainless alloy components. It has a very fine thread adjustment allowing perfect adjustments for things like rotifers-just enough to slow them down without crushing them to death. It also features an 'isolation mechanism' so that as the compressor is tightened, the top plate is not rotated in any way. This may be the best compressor ever made for this kind of work. It was not intended for high pressure compression like a trichinoscope compressor. It was patented on May 17, 1988 by Howard L. Taylor of Sarasota, Florida who studied rotifers. I do not believe these are in production any longer.