STEREOSCOPIC MICROSCOPY 



Fig. 4. The assembled zooming stereomicro- 

 scope, with built-in fluorescent lamp illuminator. 



images are always matched for power, and 

 maintain a constant focus as the image is 

 zoomed. An interesting psychological phe- 

 nomenon occurs when one zooms such a 

 microscope, in that the image definitely 

 appears to approach at high power and 

 recede at low power, despite the fact that 

 on the grounds of pure physics or geometrical 

 optics, the image lies in the same plane at 

 any power setting. A small insect placed on 

 the stage and viewed at low power assumes 

 a somewhat frightening aspect as one zooms 

 to high power, making the insect appear to 

 be a large creature approaching at a startling 

 rate of speed. 



Stereomicroscopes are supplied with a 

 wide variety of stands to hold them and 

 focus them on a great variety of objects, 

 large and small. Also various forms of illu- 

 mination are made available, such as fluo- 

 rescent lamps for illumination of transparent 

 objects or spot-light forms of illumination 



for top or oblique lighting of paque speci- 

 mens. Figure 4 shows a typical arrangement 

 for a stereomicroscope intended mainly for 

 transmitted light work. The substage illu- 

 minator contains two small fluorescent 

 lamps, with a frosted screen to even out the 

 lighting on the specimen. 



Historical Background 



Early attempts to make stereoscopic 

 microscopes concerned themselves more with 

 the binocular properties than the stereo- 

 scopic properties. Indeed many early de- 

 signs, such as those of Cherubin d'Orleans 

 (ca. 1660) were actually pseudoscopic sys- 

 tems, i.e., the left image went to the right 

 eye and vice versa. Wheatstone in 1852 

 wrote a treatise on binocular vision which 

 stimulated increased interest in the stereo- 

 microscope. Wenham, 1860, is credited with 

 the construction of the first truly stereo- 

 scopic microscope, but the forerunner of the 

 modern systems was the twin objective sys- 

 tem (Fig. 1) devised by H. S. Greenough in 

 1897. Improvements in the 1900's consisted 

 of opening up the fields of view and making 

 power change more convenient, culminating 

 in the continuously variable power system 

 introduced in 1959. 



J. R. Benford 



ENGINEERING MICROSCOPE. See LIGHT (OP- 

 TICAL) MICROSCOPY, p. 439. 



"SOLID-IMAGE" MICROSCOPE 



For many purposes, such as tracing net- 

 works of neural structure in the brain, a 

 normal microscope has the serious limitation 

 that only a thin layer of the specimen can 

 be studied at one time, owing to the limited 

 depth of focus of microscope objectives. 

 When a thin section is examined, only those 

 structures which happen to lie in the plane 

 of the section can be observed. This plane 

 may be confused by such objects as fibers 

 coming away from or toward the plane of 

 the section. A microscope which gave a large 



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