ABBE'S STEREOSCOPIC EYE-PIECE. 



of the reflecting surfaces from coming into operation, 

 \vhich can seldom be made very perfect. 



The purpose of the binocular microscope is to give 

 a stereoscopic view of objects, whereby the form, 

 distance and position of their various parts are simul- 

 taneously seen ; the result is often as striking as if the 

 minute object were placed in the hand as a model. 

 To produce a stereoscopic effect there must be an 

 equal division of the rays after they have passed 

 through the object-glass, so that each eye may be 

 furnished with an appropriate one-sided view of the 

 object ; but the methods hitherto contrived to effect 

 this not only materially injure the definition of the 

 object-glasses, but also require expensive alterations in 

 their adaptation, or, more frequently still, a separate 

 stand ; whereas the arrangement contrived by Mr. 

 Wenham is no obstacle to the use of the monocular 

 instrument, and the definition even of the highest 

 powers is scarcely impaired. Nachet of Paris has 

 throughout endeavoured to vie with Wenham, and 

 he substituted a double eye-piece for the binocular 

 body. This idea was improved upon by Tolles, of 

 Boston, U.S., and more recently it has received some 

 improvement from Professor Abbe. Fig. 71 presents 

 a sectional view of Abbe's stereoscopic eye-piece, and 

 which consists of three prisms of crown glass, a t b and 

 5', placed below the field-glass of the two eye-pieces ; 

 the tube C is slipped into the tube or body like an 

 ordinary eye-piece. The two prisms a and b are united 

 so as to form a thick plate with parallel sides, inclined 

 to the axis at an angle of 38'5. The cone of rays 

 from the objective is thus divided into two parts, one 

 being transmitted and the other reflected ; that trans- 

 mitted passing through a b and forming an image of 

 the object in the axial eye-piece B. Adjustment for 

 different distances between the eyes is effected by the 

 screw placed to the right-hand side of the figure, which 

 moves the eye-piece B', together with the prism &', in 

 a parallel direction. The tubes can also be drawn out, 

 if greater separation is required. The special feature 

 of the instrument is an ingenious arrangement for 



