PROPAGATION OF LIGHT. 



This is proved by the fact that when an opaque body is 

 interposed in the right line connecting the eye and the lumi- 

 nous source, the light of the latter is intercepted, and it ceases 

 to be visible. The same thing is proved also by the shadows of 

 bodies, which, when received upon plane surfaces perpendi- 

 cular to the path of the light, are observed to be similar to 

 the section of the body which produces them. 



This property of light was recognised by the ancients ; and 

 by means of it the few optical laws which were known to 

 them became capable of mathematical expression and reason- 

 ing. Any one of these lines, proceeding from a luminous 

 point, is called in optics a ray. 



(6) In a perfectly transparent medium, the intensity of 

 the light proceeding from a luminous point varies inversely 

 as the square of the distance. 



This is easily proved, if light be supposed to be a material 

 emanation of any kind. For the intensity of the light, received 

 upon any spherical surface whose centre is the luminous point, 

 is as the quantity of the light directly, and inversely as the 

 space over which it is diffused. But none of the light being lost, 

 the quantity of light received upon any spherical surface is the 

 same as that emitted, and is therefore constant ; and the space 



Q_ diffusion, or the area of the spherical surface, is as the square 



"*' its radius. Hence the intensity of the light is inversely as 

 9 square of the radius, i. e. inversely as the square of the 

 itance. 



Let the light be supposed to emanate from the points of > 

 uniformly luminous surface, which we shall suppose to be 

 a small portion of a sphere. Then the quantity of light 

 oliutted is proportional to the quantity emitted by a single 

 jint, and the number of points (or area) conjointly. Hence 

 a denote the area of the luminous surface, and i the quan- 

 y emitted from a single point, which is a measure of the 



B2 



