THE MICROSCOPE AND ITS REVELATIONS. 



ing a short very focus; while its top is ground flat, so that a cover 

 of thin plate-glass may be closely fitted to it by the intervention of 

 a little grease or glycerine; the whole being secured in its place by three 

 small uprights. The cell is furnished also with two small glass taps, R, 

 R, with which india-rubber tubes are connected. By this cell, which 

 may be made to serve as a moist, a warm, and a gas-chamber, experi- 

 ments on the rarefaction 

 and compression of air, 

 and on the absorption of 

 gases, can be made with 

 great facility. For l cul- 

 tivation ' experiments, 

 smaller cells are provid- 

 ed, which are attached to 

 brass-plates so arranged 

 as to have always a fixed 

 position on the stage. 1 



81. Non- Stereoscopic 

 Binoculars. The great 

 comfort which is expe- 

 rienced by the Microsco- 

 pist from the conjoint use 

 of both eyes, has led to 

 the invention of more 

 than one arrangement by 

 which this comfort can 

 be secured, when those 

 high powers are required 

 which cannot be employ- 

 ed with the ordinary 

 Stereoscopic Binocular. 

 This is accomplished by 

 Messrs. Powell and Lea- 

 land by taking advantage 

 of the fact already ad- 

 verted to ( 1), that when 

 a pencil of rays falls 

 obliquely upon the sur- 

 face of a refracting medium, a part of it is reflected without entering 

 that medium at all. In the place usually occupied by the Wenham prism, 

 they interposed an inclined plate of glass with parallel sides, through 

 which one portion of the rays proceeding upwards from the whole aper- 

 ture of the Objective passes into iheprincipal body with very little change 

 in its course, whilst another portion is reflected from its surface into a 

 rectangular prism so placed as to direct it obliquely upwards into the 

 secondary body (Fig. 57). Although there is a decided difference in 

 brightness between the two images, that formed by the reflected rays 

 being the fainter, yet there is marvellously little loss of definition in either, 

 even when the 25th-inch objective is used. The disc and prism are fixed 

 in a short tube, which can be readily substituted in any ordinary Binocu- 

 lar Microscope for the one containing the Wenham prism. Other arrange- 



Nachet's Chemical Microscope. 



1 A Mineralogical Microscope specially contrived by M. Nachet for minute 

 Petrological researches, will be described at the end of Chap. xxi. 



