26 REFLEXION AND REFRACTION. 



mentally, in the position in which the refractions are equal 

 at both sides. 



(32) We are now enabled to determine the refractive 

 index of a transparent solid experimentally. 



The first step of this process is to polish two plane faces, 

 inclined to one another at a sufficient angle, and to measure 

 that angle by a goniometer. This being done, the prism is to 

 be placed, with its refracting edge vertical, before the object- 

 glass of the telescope of a theodolite, so as to refract to the 

 cross wires in its focus the rays proceeding from a distant 

 mark. The prism is then to be turned slowly round its axis, 

 and the telescope moved, until the deviation is a minimum. 

 The horizontal circle being read, and the prism removed, the 

 telescope is to be turned directly to the distant mark, and the 

 reading repeated ; the difference of the two readings is the 

 deviation. The angle of the prism and the deviation being 

 obtained, the refractive index is given by the formula, 



sin|(A+A) 



To determine the refractive index of a fluid, we have only 

 to inclose it in a hollow prism, whose sides are formed of 

 glass plates with parallel surfaces. For the course of the ray 

 will be the same as if it had been incident directly from the 

 air into the fluid, and had emerged similarly, without passing 

 through the glass. 



(33) Let us now proceed to the physical explanation of 

 the phenomena. 



To account for the phenomena of reflexion and refraction, 

 it is supposed, in the theory of emission, that the particles of 

 bodies and those of light exert a mutual action ; that, when 

 they are nearly in contact, this action is attractive ; that, at 



