Binoculars for the Highest Powers. 217 



secured in each eye with the highest powers. These have not 

 come into use, because each image is similar, and no stereoscopic 

 appearance obtained, even by plans of illumination all effects of 

 which are identical in each. Beyond the relief afforded in the use 

 of both eyes together nothing is gained ; and I have been told by 

 some who have tried this system for difficult test observation, that 

 for perceiving delicate structure they preferred the single tube, 

 rather than endure the slight loss of definition caused by the 

 partially reflected image. 



We are thus driven back to the original principle of dividing 

 the pencil of the object-glass, and bringing the separated images 

 into each eye. 



I have before demonstrated the reason why the present binocu- 

 lars do not give a full field with powers from the ^th upwards,* 

 attributable to the distance of the prism from the back lens of the 

 object-glass. 



Immediately after this, the late Richard Beck ingeniously 

 adapted my form of prism to some |ths in the following way. The 

 prism was set just to clear the back lens, on the end of a thin elastic 

 strip or spring fixed by its upper end to the inner tube of the 

 setting. When the spring lay close to the side, the prism was 

 drawn without the pencil from the object-glass. By turning a 

 screw resting on the spring, the prism was pushed forward, so as 

 to bisect the rays. This plan was successful, and gave a clear, full 

 field in each eye-piece. It did not come into extensive use, pro- 

 bably on account of the difficulty of making prisms on such a 

 minute scale — the necessity of having one fitted to each object- 

 glass, and the inaccessible position in which they were situated. 



In all forms of binocular prisms, it is important that the 

 bisection should be fine and sharp without waste ; as any edge or 

 space at the junction will be like a thread across the centre of an 

 object-glass obscuring its most valuable portion, and in a high 

 power sadly impairing performance — knife edges are therefore 

 preferable. Nachet's equilateral prism is perfect in this respect, 

 and might easily be made as small as requisite ; but from the wide 

 angle at which the rays emerge, it could not be placed close to the 

 back lens of any present form of objective. 



Finding difficulties in setting any compact form of reflecting 

 prisms in the end of a tube sufficiently small to drop down in the 

 setting close behind the lens of a high-power object-glass, I have 

 applied the achromatized refracting prism described by me in a 

 paper read before the Microscopical Society, June 13th, 1860, by 

 which the images from the right and left half of the object-glass 

 are brought into the opposite eye with the true orthoscopic effect. 

 About this time near a dozen microscopes were fitted with these 

 * 'Quart. Journ. of Micro. Science,' April, 1861 (new series). 



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