Chap, xxviii.] BINOCULAR MICROSCOPE. 369 



seen through how many divisions of the micrometer eye- 

 piece the object extends. Suppose it is accurately 

 enclosed by t\vo divisions, then, since each equals the 

 Woo^h of an inch, the diameter of the object is the 

 smooth of an inch. 



The binocular microscope is an arrangement 

 for permitting both eyes to view the image. The 

 benefit of such an instrument is due to the fact that 

 both images will not be precisely alike. One eye 

 will receive rays which the other does not receive, 

 and the result will be the same as the effect of a 

 stereoscope, the object will be perceived in relief, 

 and elevations and depressions of the surface more 

 easily recognised. One method consists in inter- 

 cepting the rays from the objective by means of 

 two prisms, one prism deviating the rays from one 

 half of the lens, and the other prism from the other 

 half. There are thus two different tubes for such a 

 microscope, one for each set of rays. The objection to 

 this method is that the prism must be achromatic, and 

 so adds to the difficulty, while the fusion 

 of the two images gives rise to a pseu- 

 doscopic instead of a stereoscopic effect, 

 the elevations being made depressions 

 and the depressions elevations; the 

 relief is in the opposite direction to 

 what it is in reality, owing to the revers- 

 ing of the image. A method free from pr; S ms 4 '~ar- 

 these objections is shown in Fis. 164. ranged for 



mi n , , , c Bin ocular 



inree prisms are used, but they are Microscope, 

 placed not for dispersion but for 

 total reflection, dd' is the object, and rr the objec- 

 tive by which rays from dd are converged, so that 

 rays from d' entering the first prism, are totally re- 

 flected from the internal surface at u, and are 

 thrown into the second prism s' on the opposite side, 

 by whose internal surface at o' they are reflected up 

 Y 7 



