VERTICAL ILLUMINATORS AND RELATED EPISCOPIC APPARATUS

HISTORY OF ILLUMINATION OF OPAQUE OBJECTS:

In order to visualize an opaque specimen in a microscope, it must be illuminated from above, rather than below. This kind of illumination can be accomplished several ways. The simplest and earliest method was simply illumination from the side of the object. This was how all objects, transparent or opaque, were originally viewed with, e.g. tripod microscopes, until the practice of using transmitted light was used for transparent objects. This top-side illumination, is adequate with objectives of long focal length and low power, that is, 10X and lower power objectives, which with a 10X eyepiece provides a maximum magnification of about 100X, however as the power increases (and focal length decreases), it becomes more and more difficult to uniformly illuminate a specimen with ambient or even directed light. In addition, artifacts introduced by one-sided oblique illumination may appear.

A lens between the light source and the object being studied can be used to concentrate the light. Robert Hooke was one of the first to use this method. He used a hollow sphere of glass filled with water and another planoconvex lens; this lens became known as a bullseye condenser. When used from the side this was still a form of oblique illumination.

The 'bullseye' condenser, a large diameter plano-convex lens was in use for this purpose from the late 1700's onward. It could be supplied as a stand-alone lens as a 'bench' condenser, or when smaller, attached directly to the microscope via the stage, a ring around the optical tube or an extension rod attached to the limb.

With moderate powers, and also to provide uniform pandirectional lighting from above, something more than an ordinary light source from above or the side is needed. The first attempt to do this was with the use of a Lieberkuhn reflector, initially suggested by Descartes, but first known to be used on a microscope by Van Leewoenhoek about 1670, and Pierre Van Musshenbroek, as pictured in his Essai de Physique published in 1739. Although this type of reflector continued to be used by Van Leeuwoenhoek in the 2nd quarter of the 18th century, it was reintroduced and popularized in the mid to late 1700's by Johann Lieberkuhn, hence his name became associated with the device. It was soon provided with most models of pre-achromatic microscopes such as the Culpeper type,Cuff type, etc. With this device, the light source from beyond the object passes around it and is reflected back down upon it by a polished mirrored parabolic surface that surrounds the objective, with a hole in its center to allow the reflected light to pass back up into the objective. The chief limitation of this technique is that the object must be small enough to allow the light to pass around it, and this is often not the case, especially for metal samples. Once brass nosepieces became common, removeable Lieberkuhn reflectors were made to fit around the nosepiece, with the reflecting portion sitting in front of the objective. As time passed, it was recognized that different sizes and curvatures were useful for different magnifications; higher power objectives required a Lieberkuhn of smaller diameter and lesser curvature. With the advent of achromatic objectives in the 1840's individual Lieberkuhns were often supplied for each objective, fitting over each objective itself rather than both the objective and the nosepiece above.

Another method of top lighting, similar to use of a Lieberkuhn but not requiring light from beyond the specimen, is the use of a parabolic hemi-reflector. This can take the form of an attachment to the objective, or be separately mounted to be placed up against the back of the objective. Light is directed from the side of the microscope, and reflected downward upon the specimen. A modification of this device was, in addition to the reflector, was to also provide a tiny swing-in prism that can be brought outside the bottom of the objective to reflect light downward, but again, this technique is not useful with very short focal lengths, and also provides only oblique lighting.

Although episcopic illumination(illumination from above an object), originated with the very first microscopes, what we now call vertical illuminators, were first developed in the 19th century. Although with low power, top-illumination from the side can suffice, as the magnification increases and the focal length shortens, external light cannot easily be directed onto the top of an opaque specimen even with bullseye condensers and parabolic reflectors and these tend to illuminate obliquely. The vertical illuminators allow the light to come directly down onto the specimen from above usually through the objective, by reflecting incoming horizontal light off a coverslip or prism at a 45^o angle. The light reflected off the object then passes back up vertically, through the objective and the illuminator, reaching the eyepiece. With the coverslip type the reflected light goes up vertically through the angled coverslip; with the tiny prism type, it is so small, light is able to pass up the illuminator around it, but not through its center. A wider prism can also be used which is placed to one side of the illuminator, allowing the reflected light to pass up through the remaining open area.

The first idea for the use of illumination through the objective, as opposed to from outside of the objective, apparently originated with Ridell about 1865 or even earlier. It was apparently not put to practical use or produced commercially until H.L. Smith of Kenyon College published his article* in 1865. Smith's original illuminator, constructed at his request by Wales, used a reflective speculum within the illuminator to reflect light coming in from a hole on the side of the illuminator, down through the objective lenses, which was then reflected back up vertically, around the tiny speculum, reaching the eyepiece. This apparatus also had a wheel of diaphragms to regulate the aperture of light coming into it. Smith's apparatus, like that of Zeiss (see below), illuminated the object from one side of the illuminator, a semi-oblique illumination. He tried the coverslip method as in the Smith & Beck illuminator(see below), but felt the directly vertical illumination inferior to the oblique illumination. The is not surprising, as he was studying objects which were not completely flat, i.e. diatoms. Despite H.L. Smith's objections, the coverslip method became much more popular and the unrelated English firm of Smith, Beck and Beck patented a very simple coverslip vertical illuminator (at that time called an opaque or opake illuminator), in the mid-1860's.

Another method was to use a prism vertical illuminator which was inserted above the objective just like the coverslip type, but before these were developed, Tolles developed objectives with a prism built in to the objective, as shown in the JRMS in 1881(see below).

This is one of the earliest types of vertical illuminators, first patented and sold by Smith and Beck(1847-1867). It was first produced in the 1860's though it is not listed in the 1866 catalog. It uses a replaceable round coverslip (cover glass) as a reflector, while at the same time light is allowed to travel back across the coverglass to the eyepiece. In this condition, the objective acts both as a condensor and an objective. This is the very basic model with no additional components. Later refinements included a variable diaphragm to regulate the diameter of the light beam. Other refinements followed. This design, with improvements, stood the test of time and, with modifications, continued to be made well into the 20th century.

The prism-based illuminator, apparently first constructed by Robert Tolles, (Tolles' 'Interior Illuminator for Opaque Objects'), was announced in the JRMS in 1881. This model had much in common with H.L.Smith's illuminator but instead of a speculum it used a small prism. Like Smith's, it could be rotated and pushed in and out of the field. Unlike H.L. Smith's device, it was built-in to the objective rather than in a separate device above the objective. In the Tolles device the prism was placed above the most distal optical elements of the objective. The prism replaced the speculum type and competed in popularity with the coverslip model. The prism-based model was much more expensive to make however, and so its cost likely limited its popularity, the coverslip type remaining much more common.

This fine unsigned low power objective has a fixed prism below the lenses.

Also in 1881, Tighlmann reported a vertical illuminator with variable apertures, in a different configuration than Smith and Beck's. In this model, a cylinder with various different size apertures, rotated around the hole in the body of the illuminator to regulate the diameter of the light beam entering the device. Complaints were raised about this device in that making the adjustment was not as delicate as required for high power work so it was not popular. Nevertheless, Meyerowitz supplied this type of vertical illuminator in the 1890's and it is shown to the left. It was sold at the same time as their Bulloch-Myerowitz microscopes of the late 19th century.

This illuminator was first announced in the 1885 JRMS, p 522. It is nearly identical to the simpler Smith & Beck Illuminator noted above, but includes a fitting to vary the aperture to the size of one of the holes, or alternatively, the edge of the blackened brass piece can be positioned over the hole in the illuminator to create a variety of patterns of illumination. This form remained available for many years.

Powell and lealand's vertical illuminator was offered in their 1893 catalog for £ 1 10s. Similar to the Beck design, it featured a glass plate which served as the reflector and also allowed the reflected light to pass up through it to the eyepiece. It also had a wheel of apertures. The major differences from the Beck madel were that the reflecting/transmitting glass was fixed at 45 degrees, and made of thicker glass than the coverslip in the Beck version.

This is an early unsigned version of prism vertical illuminator. Nachet has been credited with developing the Prism Vertical Illuminator, sometime before 1900. This device is, like the Beck type, meant to be used installed between the nosepiece and the objective. It blocks off about 40% of the opening of the nosepiece. It is a much less expensive option than having a set of dedicated prism-equipped objectives.

A later illuminator, by Zeiss utilizes a prism as the reflector. Just like the Nachet design, this illuminator incorporates a relatively long prism so some of the light reflected back from the object is blocked. The design places the prism at the edge of the device so there is no central obstruction, at the expense of the total aperture. Later on, smaller prisms allowed more reflected light to enter the field of view, as in the prism optical illuminators Watson produced. The illuminator seen to the left was first introduced in the Zeiss catalog of 1895 and first reported in the JRMS in 1896. It continued to be offered until at least 1939. The basic version had no integrated iris or provision for dark ground illumination. A more sophisticated model had a small condensing lens and built-in adjustable iris diaphragm. At some point another model started to be produced with a slot which accomodated tin inserts with central stops, providing vertical dark ground illumination.

Watson'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). On prior coverslip models, the coverslip was easily broken; on this model a tiny frame around it reduces this problem substantially.

Starting in the 1920's much more sophisticated vertical illuminators, such as the Watson-Conrady Illuminator, came into use. This device carries the idea to a higher level of sophistication, with provision for Kohler illumination, with integrated condensing lenses, iris diaphragms, and electic lighting all built in. Not only did this make the illumination more appropriate, but because the light source was integrated into the illuminator, it eliminated the need to move the light source every time the focus was changed or the objective changed (when a traditional tube focusing arrangement was used). 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. Reichert and others supplied similar illuminators during the 2nd quarter of the 20th century.

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 through vertically towards the eyepiece. A modern form of vertical illuminator, using a coated prism is the one for the Olympus Vanox microscope in this collection.

Another way top lighting was provided, was to direct light downward around the outside edge of the optical tube, as in the Degen 'Industrial Microscope', shown to the left from about 1900. In that microscope, the moveable reflector (red arrow) could be moved up or down the outside of the optical tube to concentrate light appropriate for each objective used. This likely inspired later microscopes in which the light was directed around the outside of specially designed 'Epi' objectives, where the curvature of the light-director could be shaped and focused optimally for each of the different power objectives. Zeiss was apparently the first company to manufacture epi objectives. This collection includes a Wild Epi-illumination system.

*Smith, H.L. Art. XXIV.-On a new Illuminator for Opaque objects under high powers of the Microscope; Am J of Science and Arts Second series. Vol XL July-Nov 1865 pp238-41.