Ch. ni] BINOCULAR MICROSCOPES 91 



continued observation. The stereoscopic appearance, while desirable 

 in some cases, is not of very much advantage in revealing structure 

 when working with high powers. 



§ 161. Experiments with single objective binoctxlars. — So far 

 as the hghting is concerned it is exactly as for a monocular microscope 

 (§83-130). ■ 



§ 152. Arrange the microscope in a convenient position, use any 

 pair of oculars (sx or lox) and any objective, but to begin with a 

 low-power (16 mm.) objective is to be preferred. Use a preparation 

 like the mounted fly's wing or a preparation showing injected blood 

 vessels. Look into the microscope and arrange the hght so that the 

 object is well illuminated. The determination of binocular vision 

 is exactly as with the double-objective binocular (§ 147). 



§ 153. Pupillary distance. — To vary the distance of the eye-points 

 for converging tubes rotate the oculars equally up from the lowest 

 point until the binocular efiect is secured, and then note the position 

 of the oculars. If the tubes are parallel there is a screw between them 

 by which they can be separated or approximated. Continue to adjust 

 untU the binocular vision is perfect and then note the position so 

 that the tubes can be set in the right position instantly at some 

 future time. Each individual must determine the pupillary distance 

 of his own eyes. The chances are against any two persons being alike 

 in that respect. 



§ 154. Unlikeness of the two eyes. — With many persons the 

 refraction of the two eyes is somewhat different, so that if the micro- 

 scope is in focus for one eye it is necessary to refocus for the other. 

 Now if this is the case it is necessary to focus the two tubes differently 

 in a binocular. Focus first with the tube which is not adjustable for 

 parallel tubes, or with either one where converging tubes are used. 

 Then close the eye used for focusing first and focus the other tube for 

 the other eye by rotating the tube up or down until the image is sharp,, 

 or by turning the milled ring in the adjusting tube of the parallel tube 

 type. If one now looks into the microscope with both eyes there wiH 

 be two sharp images fused. 



