9 8 



VISION WITH THE COMPOUND MICROSCOPE 



Nachet's Binocular was early in the field, but was riot a 

 practical construction on account of the parellelism of its tubes, ;nul 

 is not now advocated by its inventor or adopted by opticians of any 

 country. 



Wenham's Stereoscopic Binocular. All these objections are 

 overcome in the admirable arrangement devised by the ingenuity of 



Mr. Wenham, in 1860 (Trans. Microscopi 

 cal Soc. of London, vol i. N.S. p. 15), in 

 whose 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.'!. BO 

 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 6, going on continuously to the eye- 

 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. 73, 74, a), on its entrance into the 

 prism, is scarcely subjected to -mv refrac- 

 tion, since its axial ray is perpendicular 

 to the surface it meets ; but within the prism it is subjected to two 

 reflexions at b and c, which send it forth again obliquely in the line 



K 



FIG. 73. Wenham's prism 

 (1860). 



FIG. 74. FIG. 75. 



Wenham's stereoscopic binocular microscope (1860). 



(I towards the eye-piece of the secondary or left-hand body (fig. 74. 

 L) ; and since at its emergence its axial ray is again perpendicular 



