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VAN HEURCK MICROSCOPES (Vertical Illuminators)

19th C through 1920's

Makers: Beck, Zeiss, and W. WATSON & SONS LTD.

beck Among the accessories with the Watson Grand Van Heurk microscope were five vertical illuminators. One is the early brass type with a coverslip used as the reflector, which was patented by Beck and made since about 1866.

Watson VI This vertical illuminator came in a can engraved on the lid: 'VERTICAL ILLUMINATOR, W.Watson & Sons Ltd., 313 High Holborn, London'. It was first announced in the 1885 JRMS, p 522. It is an improvement in the Beck version, offering a total of three different aperture diameters. The largest is used by pushing the diaphragms aside and is simply the hole in the illuminator. The triangular piece offers two other smaller apertures to choose from. This improvement in the Beck model was the earliest to offer different apertures for the entering light built in to the illuminator. Later, more sophisticated designs eventually included a miniature continually adjustable iris diaphragm. A further improvement was to include a built in lens to concentrate the light onto the reflector. Both these features are incorporated into the Watson Combined Vertical Illuminator shown below.

zeiss vert ill A third, later illuminator by Zeiss, utilizes a prism as the reflector. The design of this illuminator incorporates a relatively large prism so some of the light reflected back from the object is blocked. In addition to their own products, Watson offered some by Zeiss in the early 20th century. Later on, smaller prisms allowed more reflected light to enter the field of view, as in the prism optical illuminators Watson themselves produced. The Watson 'Combined Vertical Illuminator' also used a small prism.

watson combined vert illWatson's 'Combined Vertical Illuminator' set, in a signed case, provides the option of a tiny prism or coverslip for reflecting the illuminating light beam. It also had a condensing lens and an iris diaphragm to regulate the light directed through it. The Watson 'Combined Vertical Illuminator' was first offered by Watson in the 1920's after 1921, and is shown in the 1926 catalog (thanks to Dr Joe Zeligs for this information).

Finally a 'Watson-Conrady Condensor Vertical Illuminator' with built-in light source is also present. It could be powered by a battery, or with house current, with a resistance device. In addition to a lens system, it had two iris diaphragms making Kohler illumination possible. Furthermore, because the light source was intrinsic to the illuminator, it could be used with a tube-focusing microscope without the annoyance of having to move the light source to adjust for changes in focus or objective. This set the standard for modern vertical illuminators, providing Kohler illumination in the vertical illumination configuration. The Watson-Conrady Condenser Vertical Illuminator was offered from about 1923 through the 1940's.

The very first microscopes used only top lighting. These included tripod microscopes with a flat opaque stage. Soon however, this was insufficiently bright. At first this problem was addressed by using a device to concentrate light on the specimen; these often used a lens or lens system to concentrate the light from the side above the specimen. The large single 'bullseye' condenser was used for this purpose for centuries. It could be freestanding as in a 'bench' condenser, or come off an adjustable arm pivoting off the stage, arm or even main optical tube of the microscope. When electric lights became a common illumination source, the bullseye lens was eventually built in to them.

leibAnother method of illuminating an opaque object was the Lieberkuhn. This idea dates back to Descartes in 1637 and also to Van Leeuwenhoek. But other than by Van Leeuwenhoek, the idea was not pursued much, until Johannes Lieberkuhn popularized the idea in Europe and then England in about 1738. It was John Cuff who was apparently the first in England to latch on to this idea and by the early 1740's was producing an improved compass microscope he called his 'Microscope for Opake Objects'. A slightly later example of the Lieberkuhn compass microscope (c. 1800) is shown here to the left. This speculum device surrounds the objective. Light is directed from behind the object, (or below the stage in microscopes with a stage), passes around the opaque object, and is then reflected back onto the object by the Leiberkuhn. Cuff also incorporated Leiberkuhns into his 'Ellis Aquatic' simple microscopes and on the nosepiece of his 'Cuff-type' compound microscopes. Leiberkuhn reflectors designed for individual objectives were supplied by most manufacturers well into the 1880's.

As time went on, and as the optics improved, and shorter focal length objectives of higher power became common, lighting from the side became inadequate and Lieberkuhn reflectors were impractical for larger objects or surfaces. In order to use a Lieberkuhn the subject has to be small enough for the light to pass around it and still reflect off the speculum. Vertical illuminators could work with any size object.

Vertical illuminators are used for illumination of solid or opaque objects with higher power objectives. Although with low power, top-illumination from the side can suffice, as the magnification increases and the focal length shortens, light cannot easily be directed onto the top of an opaque specimen.

The vertical illuminators allow the light to come directly down onto the specimen from above through the objective, by reflecting incoming light off a coverslip or prism at an angle (usually 45o) which at the same time allows the reflected light to pass vertically back up through the coverslip or around the prism upwards towards the the eyepiece.

Early versions were simply a tube with a hole and an angled coverslip inside. As time went more features were added such as the capacity to regulate the light entering and eventually built-in condensing lenses as shown here to the left. With the coverslip type, the light enters from the side, is reflected down on the object being studies, and goes up vertically through the coverslip. A tiny prism could also be used, but in this case it is so small, light is able to pass up the illuminator around it, rather than through its center. These arrangements both have disadvantages but the Watson-Conrady type, a predecesor of the modern type of vertical illuminator, provides much better illumination of opaque objects. Another variation of vertical illumination is to direct light vertically down upon the object via a reflector around the outside of the objective, or in an earlier version, around the optical tube. In the modern form, in a process known as 'epi-illuminiation' the light to the reflector is supplied by a sophisticated light path from above the reflector, obviating the problems with Lieberkuhn reflectors and allowing any size object to be examined. An earlier type, using a reflector around the optical tube, can be seen on the Degen Microscope in this collection. An example of the epi type of illumination can be seen in the Wild Epi-illuminator in this collection which uses specialized objectives, each with an optimized light path and reflector which surrounds the objective. An example of a modern microscope in this collection using a vertical illuminator similar to the Watson-Conrady device is an Olympus Vanox microscope with vertical illumination. That instrument has capabilities for not only brightfield vertical illumination, but also polarized light illumination and even interference contrast. Modern vertical illuminators are similar to the Watson-Conrady model except instead of a coverslip as reflector, they use a beam-splitting prism, which allows highly reflected light to illuminate the object while at the same time allowing most of the light bouncing off the object to pass back vertically through the beam splitter towards the eyepiece.