422 PHOTOMETRIC OBSERVATIONS. 



another luminous body which is at a less distance from 

 the surface, unless it gives out much more light, or, as 

 it is termed, has a greater illuminating power than the 

 nearer source of light. Since the intensity of the light 

 at twice the distance is one -fourth, arid at three times 

 the distance one-ninth, it follows that if two sources 

 of light, of which one is placed at a certain distance 

 from a surface, while the other is placed at a distance 

 twice or three times as great, produce equal degrees 

 of illumination, the illuminating power of the more 

 distant body must be four or nine times as great com- 

 pared with the illuminating power of the source of light 

 which is nearer to the surface. Hence, when two 

 sources of light produce equal intensities of light upon two 

 surfaces at unequal distances, their illuminating powers 

 are in the ratio of the square of their distances from the 

 illuminated surfaces. 



This law is applied in 'photometry/ that is, the 

 measurement of the relative intensity of different 

 sources of light. There are various kinds of 'photo- 

 meters ' used for this purpose. 



Eumford's Photometer (fig. 241, A), consists of a 

 screen, in front of which is fixed an opaque rod ; the 

 lights to be compared for instance, a lamp and a candle 

 Zj and Z 2 , are placed at a certain distance, in such a 

 manner that each projects on the screen a shadow of 

 the rod, Si and S 2 , close to one another. The distances 

 of the sources of light are adjusted so that both 

 shadows appear equally dark. The illuminated portion 

 of the screen receives light from both sources ; the 

 shadow Si produced by the lamp LI receives light from 

 the candle L 2 only, while /Sj, the shadow produced by 



