356 OSCAR W. RICHARDS 



objectives and the oculars are the same and the eyes are placed at the 

 unit plane of the instrument, the image is orthostereoscopic. How- 

 ever, microscopists often prefer to have the depth emphasized and 

 the convergence of the objectives is increased to give hj^perstereos- 

 copy. While in general this is helpful, some distortion does occur 

 and should be considered in interpreting the image. 



The limitation of the instrument arises from the mechanical dif- 

 ficulty of placing the front lenses of the two objectives close together, 

 resulting in the numerical aperture's being limited to about 0.12. 

 With the working rule that for the average eye the useful magnifica- 

 tion is 1000 N.A., there is no need for greater magnifications than 

 about 120 to 150 X. In fact they are most useful for specimens seen 

 at 5 to SOX magnification, because at the lower magnifications more 

 of the specimen can be seen at one time. 



The instruments have means for moving the oculars to accommo- 

 date the separation of the observer's eyes; also, one eyepiece is usu- 

 ally separately focusable for individuals with unequal eyes. The in- 

 strument is focused with a simple rack and pinion since a fine adjust- 

 ment is unnecessary at these low magnifications. No special skill is 

 required of the user. It is well to check the instrument occasionally 

 by looking at a small ruler placed horizontally and then vertically 

 across the field of view. The edges of the field for each eye (seen when 

 the other is closed) should be the same. If not, the instrument 

 should be corrected by the maker or by a competent person. With a 

 comfortable viewing position and adequate, but not too bright, il- 

 lumination the microscope can be used for considerable periods of 

 time without strain. 



3. Vertical Illumination 



The vertical illuminator (Fig. 4) uses a half-silvered cube or a 

 coated thin, flat piece of glass mounted at the back of the objective 

 to reflect light, from a source at the side, through the objective to the 

 specimen. Light returning from the specimen passes through the re- 

 flector to the eye. A totally reflecting prism, small enough to cover 

 only part of the aperture, has also been used. Vertical illuminators 

 are useful with polished metal surfaces and other specimens of high 

 reflectivity. Some glare results since the light goes through the ob- 

 jective in both directions. For best observation antireflection coat- 

 ings are essential for all glass-air surfaces to reduce the glare. When 



