4 PROPAGATION OF LIGHT. 



absolute brightness, the intensity of the illumination, at any 

 distance d, is 



ai 



# 



(7) A plane surface, whose dimensions are small in com- 

 parison with the distance, and which is perpendicular to the 

 incident light,- may, without sensible error, be considered as a 

 portion of a spherical surface concentric with the luminary. 

 The intensity of the illumination, therefore, or the quantity of 

 light received upon a given portion of such a plane, is expressed 

 by the formula of the preceding Article. 



When the surface is inclined to the incident light, the 

 quantity of the light received by any given portion is dimi- 

 nished in the ratio of unity to the sine of the angle of inclina- 

 tion. The intensity of the illumination is, therefore, diminished 

 in the same proportion, and is expressed by the formula 



ai sin 9 



~ir~' 



9 being the inclination of the surface to the incident light. 



(8) Experience proves that the eye is incapable of com- 

 paring directly two lights, so as to determine their relative 

 intensities. But, although unable to estimate degrees, the eye 

 can detect differences of intensity with much precision ; and 

 with this power it is enabled (by the help of the principles 

 just established) to compare the intensities of two lights indi- 

 rectly. 



Let two portions of the same paper, or any similar reflect- 

 ing surface, be so disposed, that one of them shall be illumi- 

 nated by one of the lights to be compared, and the other by 

 the other, the light being incident upon each at the same 

 angle. Then let the distance of one of the lights be altered, 

 until there is no longer any appreciable difference in the inten- 



