PROPAGATION OF LIGHT. O 



sities of the illuminated portions. The illuminating powers of 

 the two lights will then be as the squares of their respective 

 distances ; and their absolute brightnesses as the illuminating 

 powers directly, and as their luminous surfaces inversely. For, 

 if i and i' denote the absolute brightnesses of the two lights, 

 a and a' the areas of the luminous surfaces, and d and d' their 

 distances from the paper, the intensities of illumination are 



ai sin , a'i' sin , . , , . 



and , respectively ; and these being rendered 

 d d 



equal in the experiment, we have 



ai d? 



The following simple and convenient mode of practising this 

 method was suggested by Count Eumford. A small opaque 

 cylinder is interposed between the lights to be compared and 

 a screen ; in this case it is obvious that each of the lights 

 will cast a shadow, which is illuminated by the other light, 

 while the remainder of the screen is illuminated by both lights 

 conjointly. If, then, one of the lights be moved, until the 

 shadows appear of equal intensity, their illuminations are 

 equal, and, therefore, the illuminating powers of the two 

 lights are to one another as the squares of their distances 

 from the screen. 



(9) Light is propagated with a finite velocity. 



This important discovery was made in the year 1676, by 

 the Danish astronomer, Olaus Hoemer. Roemer observed that 

 when Jupiter was in opposition, and therefore nearest to the 

 Earth, the eclipses happened earlier than they should according 

 to the astronomical tables ; while, when Jupiter was in con- 

 junction, and therefore farthest, they happened later. He 

 thence inferred that light was propagated with a finite velo- 

 city, and that the difference between the computed and 



