Testing a Microscope, 33 



rotates at all, or a movement of it can be detected with- 

 out a corresponding motion on the movable part, there 

 is backlash. Then, there should be no shake in any of the 

 fittings. In a badly-constructed microscope, even when the 

 fittings are in their most advantageous position, by holding 

 them and shaking them slightly a movement in the slides 

 can be detected. The body should be racked up a consider- 

 able distance to see whether any rock or shake beyond that 

 of the tension on the bearings can be detected. An instru- 

 ment sound in construction should exhibit none whatever. 

 The stage should be treated in the same way. A good idea 

 of the comparative quality and finish can often be obtained 

 by examining some hidden or unnoticed part, and observe 

 whether the same care in finishing has been exercised there 

 as in parts that are seen. For instance, if some micro- 

 scopes be examined underneath the foot, they will be found 

 left in the rough as cast, and merely blacked over ; while 

 another instrument will be found carefully finished in that 

 part. It does not necessarily follow that the former is a 

 bad instrument, but it would often be found, if taken to 

 pieces, that there was not a careful fitting in working parts 

 that did not catch the eye, and a probability of its not being 

 so durable as the better-finished instrument. 



BINOCULAR MICROSCOPES. 



We have hitherto been treating principally of the 

 monocular microscope, and this, it must be understood, is 

 the only form that can be used for critical high-power work 

 in fact, the Continental firms as a rule do not make 

 binocular microscopes at all, regarding them as un- 

 necessary. Two or three of them, however, make a 

 binocular eyepiece, which will be found described under 

 the head of eyepieces. The advantage of a binocular 

 microscope is that both eyes can be employed simul- 



