THE BINOCULAR MICROSCOPE. 115 



of the reflecting surfaces from coming into operation, 

 which can seldom be made very perfect. 



" The definition with these prisms is good ; but they 

 are liable to objection on account of the extremely small 

 portion of the field of view that they take in, and which 

 arises from the distance that the eyes are of necessity 

 placed beyond the focus of the eye-piece, where, the rays 

 being divergent, the pupil of the eye is incapable of taking 

 them all in; also there is great nicety required in the 

 length of the prisms, which must differ for nearly every 

 different observer." 



The great disadvantages of the first arrangement of the 

 binocular microscope were the expensive alterations 

 required in their adaptation : to most persons, the view 

 that it gave of the object was pseicdoscopic, and not that 

 of solidity and roundness ; and the two bodies being 

 united at a fixed angle of convergence, the distance be- 

 tween their axes could not be adapted to the varying 

 distances between the eyes of different individuals. At 

 length, these, as well as other defects, have been com- 

 pletely overcome by the improvements the instrument 

 has recently received at the hands of the inventor; and 

 we have no hesitation in saying that Wenham's prism is 

 the most valuable addition the microscope has received 

 since the perfection of the object-glasses. The adaptation 

 does not at all interfere with the use of the instrument 

 as a monocular microscope, and such additions as the 

 microspectroscope can be as easily used with it as in the 

 old form ; it also affords a ready comparison between 

 the object as seen singly, and by natural double vision, 

 and thus may be obtained an ever ready test, or sight 

 analysis of the structure under examination. Besides, 

 the relief afforded to the eyes is, to our sensation, 

 something quite marvellous, and we believe, therefore, 

 much less risk is run of doing injury to vision. Objects 

 become easily distinguishable, more especially all opaque 

 or semi-opaque ones ; and for the more transparent ones 

 all that is required is to use care in their illumination — 

 diffuse the light by placing a piece of ordinary tracing or 

 tissue paper, or ground glass, between the light and the 

 object ; or even polarised light for peculiar substances will 



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