Management of Light in Illumination. 175 



direct rays of the flame, and by preventing its being 

 deranged by them greatly facilitate distinct vision. 



In order to be able to form a just idea respecting 

 the manner in which light is dispersed in passing 

 through ground glass and other like substances, it may 

 be useful to examine the matter with some attention ; 

 and, as the laws which govern the rays of light in 

 their passage through diaphanous bodies are perfectly 

 known, there is no difficulty whatever in explaining 

 the phenomena in a manner which will be perfectly 

 satisfactory, even to those, I trust, who have not made 

 the science of optics a part of their studies. 



Light always passes from luminous bodies in straight 

 lines, and continues to move on in the same direction, 

 without deviation, except when it is reflected or when 

 it is refracted, or drawn out of its straight course, in 

 passing out of one transparent substance into another. 



When a ray of light, in passing out of the air into 

 glass, strikes the glass in a direction which happens to 

 be exactly perpendicular to that part of the surface of 

 the glass where it arrives, it enters the glass without 

 being at all drawn aside or deranged in respect to the 

 direction of its course, and it continues to move on in 

 the glass in the same straight line ; and, farther, if the 

 ray in passing out of the glass happens to arrive at a 

 part of the surface of the glass which is perpendicular 

 to the direction of its course, it will pass directly 

 through it also, and continue its course in the air in 

 the same direction in which it moved before it arrived 

 at the glass. 



But when a ray of light in entering glass (or any 

 other transparent substance) meets with a surface 

 which is not perpendicular to the direction in which 



