l66 PROPERTIES OF LIGHT. 



quantity of ligiit refracted extraordinarily is proportional to 

 the square oi' the sine of the same angle. If it be the ex- 

 traordinary ray of the first crystal on which we operate, we 

 obtain a similar result, changing the word ordinary for ex- 

 traordinary, and reciprocally. 



With regard to reflection, if v/e consider, for example, a 

 ray reflected by one glass, with which it makes an angle of 

 Keflcctiun. 35° 25 , and falling on a second glass at the same angle, 

 the angle comprised between ihe two surfaces being in other 

 respects arbitrary : we must conceive a plane perpendicular 

 to the first glass, and another perpendicular to the second, 

 passing through this reflected ray; and the quantity of 

 light reflected by the second glass will be proportional to the 

 square of the cosine of the angle comprised between these 

 two planes. 



I shall confine myself to a few examples of the application 



of this law. 



Examples of Whei! a ray is reflected by the surface of a glass at an 



thenij iicnticn angle of 54" 35', we find, that all its particles are disposed 

 of this law. ^ . .„ T 1 , 



Example 1. in the same manner ; since, if we present perpendicularly to 



this ray a prism of crystallized calcareous spar, the axife of 

 which is in the plana of reflection, all its particles will be 

 refracted in a single ordinary ray, none being refracted ex- 

 traordinarily. In this case the analogous axes of these par- 

 ticles are all parallel, since they all cou)port themselves in 

 the same manner. Let us call the axis of these particles, 

 which are perpendicular to the plane of reflection, b. All 

 the particles, of which the axis c was perpendicular to that 

 plane, have penetrated the transparent body. If therefore 

 we present to the particles reflected, and under the same 

 gnifle, a second glass parallel to their axis c, they will be 

 found similarly circumstanced with those, which could not 

 be reflected by the first; the ray therefore will penetrate 

 this second glass entirely. In fact, experiment shows, that, 

 under these circumstances, all the particles escape the 

 forces of reflection. 



When we place two rhomboids of calcareous spar on one 



another, so that their principal sections are parallel, a sola? 



^ Example 2. yaj' parallel to these priiicipal sections produces but two 



emergent 



