microscopes are monocular, meaning that there is only one tube, one objective, and one ocular. Petrographic micro-scopes, used for the examination of thin sections by transmitted light, are also of this design (Fig. 13-1).
Figure 13-2 illustrates the basic monocular microscope setup. Below the stage is a light source with a field con denser and iris diaphragm, both of which are used to control the field illu-mination. Immediately below the stage is a substage condenser and substage diaphragm that control image contrast,
Figure 13-4 Binocular Greenough microscope by Bausch & Lomb. (jeff Scovil Collection)
i mage diffraction, and depth-of-field.
Figure 13-3 shows an antique monocular microscope.
Except for petrographic microscopes, monocular microscopes are of little use in the fields covered in this book. There are several reasons for this: (1) they are too high powered for most of our pur-poses, (2) they are designed primarily for transmitted light viewing of transparent
subjects, and (3) they are designed pri-marily for viewing what are essentially
"flat" objects, while we are interested pri-marily in three-dimensional objects.
BINOCULAR/
STEREOMICROSCOPES
The simplest stereomicroscopes have two tubes, each with its own ocular and objective. They are essentially two monoculars mounted side-by-side at an angle of 12-14°. They are angled so that they focus on the same subject, albeit each with a slightly different view. The result of this configuration is a threedimensional view of the subject. Such microscopes are called Greenough Microscopes and are relatively low powered (usually less than 100x). They are used primarily for viewing threedimen-sional subjects that may be opaque (Figs.
13-4 and 13-5). The wide-field micro-scope has separate oculars, but uses only one objective ( Fig. 13-6). For a more detailed discussion on these two types of microscopes, see Wight (1993). Gemological microscopes are stereo microscopes fitted with forceps for holding cut stones, plus substage condensers, iris diaphragms, and dark-field, reflected, and transmitted lighting(Fig. 13-7).
For best photographie results, both the oculars and objectives should be the same brand and designed for use with each other. They should also be fíat-field lenses so
that all will be in focus on the film plane. Many lenses are not fíat field and, while suitable for direct viewing, are not good for photography.
11 6
Eyepiece
Prism
Objective lenses
Minor
Eyepiece
Prism
Zoom mechanism
Single objective lens
Figure 13-5 The Greenough binocular micro-scope is essentially two joined monocular microscopes. (Wight, 1993)
Figure 13-6 The wide-field microscope uses a single large objective lens. (Wight, 1993)
TRINOCULAR
PHOTOMICROSCOPES
This type of microscope has the two standard tubes, as well as a third designed solely for use in photography (Fig. 13-8). There are several advantages to the use of such a microscope. (1) The third tube allows viewing with both eyes while the camera is mounted. (2) The photo tube is usually of a larger
diame-ter, reducing potential reflections from the use of smaller diameter viewing tubes. (3) The photo tube is a straight line to the objective, without the prisms in the viewing tubes that can degrade the photographic image. (4) If the objective for the photo tube is separate from the view-ing tubes, they are probably optimized for photographic work, producing a superior image.
There are several ways that light is channeled into the photo tubes:
1. The photo tube has its own separate, complete optical system. This arrangement is simple but fairly expensive. It has, howcver, the advantage of always being available and has the same brightness as the oculars. Besides the added expense, a disadvantage is its slightly different view than the viewing tubes.
2. A beam-splitter can be used in one tube to divert the light from one ocular. The light is always available and has the same view as the oculars, but there is a reduced amount of light to each (Fig. 13-6).
3. A slide mirror or prism diverts the light from one of the oculars. This method affords the same orienta-tion and amount of light, but when in use, the ocular from which the light is diverted cannot be used. The mirrors that must be used are front surfaced to minimize distortion. The coating on such mirrors is deli-cate and subject to discolorization after a few years.
Figure 13-7Gemological microscope with both Figure 13-8 Russian MBS-10 trinocular micro-transmitted and reflected illumination, and dark- scope with photo tube on right. The light beam field lighting (Courtesy of Michael's Creative i s diverted by beam-splitters that are placed in Jewelry) the light path by means of a lever. (Courtesy of
C & N Minerals)
Many trinocular microscopes have an iris diaphragm built into the photo tube for depth-of-field control, and the tube can be extended for aid in focusing. Some have a right-angle eyepiece built into the tube for viewing with the camera mount-ed. Another advantage of the photo tube is that you can make the camera parfocal with the oculars, that is, in focus in the same plane of the subject at the same time.
Higher powered monocular photomi-croscopes have some advantages over trinocular photomicroscopes because of the following additional accessories to aid in photography: mechanical stages for
precise manipulation of the subject; sub-stage condensers that can be focused, centered, and rotated; iris diaphragms;
and provisions for mounting filters, cen-ter stops, and other light modifiers.
Although trinocular microscopes do not have all of these accessories, they still are the best means of doing photomicrography, although relatively expensive. Most of the microscopes in use today are of the stan-dard stereo variety. There are few people willing to invest in a second, more expensive microscope just for the sake of photography.
While using a standard stereo microscope for photography is not as convenient or up to the same standards of quality, very good 118
Figure 13-9 Setup for photomicrography on an optic bench with lens on camera. (Wight, 1993)
results can be obtained. It is, of course, true that the higher the quality of the instru-ment, especially the optics, the better the
quality of the photomicrograph.