9 s 



VISMN WITH THE COMPOUND MICROSCOPE 



Nachet s Binocular wa.-> early in the field, but was not a 

 practical construction on account of the parellelism of its tubes, and 

 i- not now advocated by its inventor or adopted by opticians ot'anv 

 country. 



Wenham's Stereoscopic Binocular. All these objections are 



overcome in the admirable arrangement devised by the ingenuity of 



Mr. Wenham, in I860 (Trans. Microscopi- 

 cal Soc. of London, vol i. ]S~.S. p. !."). in 

 wlio>e binocular the cone of rays pro- 

 ceeding upwards from the objective is 

 divided by the interposition of a prism 

 of the peculiar form shown in fig. 7-!. so 

 placed in the tube which carries the objec- 

 tive (figs. 74, 75, a), as only to interrupt 

 one half, a c, of the cone, the other half, 

 a b, going on continuously to the eve- 

 piece of the principal or right-hand body, 

 R, in the axis of which the objective is 

 placed. The interrupted half of the cone 

 (figs. 7,'}, 74, .), on its entrance into the 

 prism, is scarcely subjected to any refrac- 

 tion, since its axial ray is perpendicular 

 to the surface it meet.-, ; but within the prism it is subjected to two 

 reflexions ai // and c, which send it forth again obliquely in the line 



1 1. K 



FIG. 73. Wenham's prism 

 (I860). 



FIG. 74. FIG. 75. 



^ enhnjn copic Iminrular microscope i IM;U 



''" eye piece of the secondary or left-hand body (fiy. 74. 

 at its emergence it - axial ray is again perpendicular 



