120 



PRACTICAL TREATISE ON THE MICROSCOPE. 



m, instead of passing on in a straight line to the object, i i, are 

 converged by the first surface, b a, upon the oblique plane, r ; 

 from this they are reflected to v, where they receive a second 

 reflection, and are finally converged by the convex surface, 

 c d, upon an object, i i. This prism is 

 set obliquely in a tube of brass, and 

 should be so contrived that it may be 

 revolved, in order that the effect of 

 oblique light may be shewn upon all 

 parts of an object. In the microscope 

 of M. Nachet, the stage can be re- 

 volved, but in all our English instru- 

 ments, except those provided with a 

 stage, such as that proposed by Mr. 

 Legg, the prism itself must be turned. 

 Mr. Shadbolt has given the curves 

 which answer best for the prism, in a 

 paper in the third volume of the Trans- 

 actions of the Microscopical Society. In 

 the prisms first supplied by M. Nachet, 

 the angle was 30°, and both upper and 

 under surfaces were convex ; he now 

 makes the lower surface plane, and, as 

 it turns out, the plan for some time 

 adopted by M. Nachet is precisely that determined mathe- 

 matically by Mr. Shadbolt. 



The condensers of M. Nobert, Mr. Shadbolt, and Mr. 

 Gillett, together with a prism of Amici, and some other 

 equally useful pieces of apparatus concerned in the illumi- 

 nation of objects, will be described in Part II., relating to the 

 " Use of the Microscope." 



Polarizing Apparatus. — This consists of two prisms of cal- 

 careous spar, constructed after the plan of Mr. Nicol, of Edin- 

 bm-gh, and composed each of two pieces of the same spar, 

 cemented together so as to transmit a single image only. One 

 of these is mounted in a tube, and adapted to a flat plate of 

 brass, as represented by fig. 66, by which it can be applied to 

 the under surface of the stage-plate, like the achromatic con- 



Fig. 65. 



