THE BINOCULAR MICROSCOPE. 



169 



under- surface of the flint-prism. On leaving the second 

 surface, and entering the crown-prism, it is slightly bent 

 inwards, and on finally emerging, it is refracted outwards 

 in the direction required. The base of the compound 

 prism should not be larger than is sufficient to cover the 

 stop of the lowest object-glass, in order that they may be 

 made very thin. 



" The method of applying the prism to the binocular 

 microscope is shown in fig. 98 : a a is the object-glass ; 

 5, the prism placed as close be- y 



hind it as the fittings will admit. 

 The prism is set in an aperture 

 in a flat disc of brass, which has 

 a horizontal play in every direc- 

 tion, in order that it may be ad- 

 justed and fixed in such a position 

 that the junction of the prisms 

 may bisect the rays from the ob- 

 ject-glass, and at the same time 

 be at right angles to the trans- 

 verse centres of the eye-piece ; c c 

 are the two bodies of the micro- 

 scope, provided with the draw- 

 tubes and the usual eye-pieces d d. 

 The distance between them should 

 be rather less than the average 

 distance asunder of the eyes ; and 

 in cases where these are very wide 

 apart, we can pull out the draw- 

 tubes, which will increase the 

 distance between the eye-pieces. 



"With this apparatus I ob- 

 tain the whole of the field of Fig - 98 ' 

 view in each eye ; which circumstance I was not prepared 

 to expect, as this must, in some measure, depend upon 

 the correction of the oblique pencils of the object-glass, 

 for we cannot expect to look obliquely through the objec- 

 tive of a compound achromatic microscope in the same 

 way as in the single lens arrangement, but can only avail 

 ourselves of such oblique pencils of rays as are corrected 

 for passing through the axis of the microscope." 



