32 



OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Fig. 18. 



separated-from or approximated -towards each other, so that the 

 distance between their axes can be brought into exact coincidence 

 with the distance between the axes of the Eyes of the individual 

 observer. — This instrument gives true Stereoscopic projection to the 

 conjoint image formed by the mental fusion of the two distinct 

 pictures ; and with low powers of moderate angular aperture its 



performance is highly 

 satisfactory. There are, 

 however, certain draw- 

 backs to its general 

 utility. First, every 

 ray of each pencil suf- 

 fers two reflexions, and 

 has to pass through 

 four surfaces ; this 

 necessarily involves a 

 considerable loss of 

 light, with a further 

 liability to the impair- 

 ment of the image by 

 the smallest want of 

 exactness in the form 

 of either of the prisms. 

 Second, the mechanical 

 arrangements requisite 

 for varying the dis- 

 tance of the bodies, in- 

 volve an additional 

 liability to derange- 

 ment in the adjust- 

 ment of the prisms. 

 Third, the instrument 

 can only be used for 

 its own special pur- 

 pose ; so that the ob- 

 server must also be 

 Cachet's Stereoscopic Binocular. provided with an or- 



dinary Monocular Microscope, for the examination of objects un- 

 suited to the powers of his Binocular. Fourth, the parallelism 

 of the bodies involves parallelism of the axes of the observer's Eyes, 

 the maintenance of which for any length of time is fatiguing. 



27. Wenham's Stereoscopic Binocular. — All these objections 

 are overcome in the admirable arrangement devised by the 

 ingenuity of Mr. Wenham, the advantages of which are so great 

 that no Microscope can now be considered complete which does not 

 possess it. — In Mr. "Wenham's Binocular the cone of rays pro- 

 ceeding upwards from the Objective is divided by the interposi- 

 tion of a Prism of the peculiar form shown in Fig. 19 ; this is so 



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