424 BUNSEN'S PHOTOMETER. 



grease-spot is made, is held before a window, or, in the 

 evening, before a lamp, so that the paper is illuminated 

 from behind, the spot appears light on a dark ground, 

 for the greased portion transmits more light than the 

 surrounding part of the paper. If, on the contrary, 

 the paper be illuminated by light placed in front of it, 

 the spot will appear darker than the surrounding part 

 of the paper : the greased portion allows more light to 

 pass than the ungreased portion, and therefore the latter 

 reflects more light, in other words it appears brighter. 

 If the paper be held nearly with its edge towards the 

 window or the lamp, and slightly turned to arid fro, a 

 position will be found in which the greased part and the 

 rest will appear almost alike : when this is the case, 

 the intensity of the illumination on both sides is the 

 same. 



Bunseris Photometer (fig. 242), depends upon this 

 principle. A paper screen, in which a circular grease 

 spot is made, is placed in the straight line between the 

 lights to be compared, and moved in this line to and 

 fro until the grease-spot disappears. It is now only 

 necessary, as in Eurnford's photometer, to measure the 

 distance of each light from the illuminated screen, and 

 the ratio of the squares of these distances will give the 

 ratio of the illuminating powers of the lights used. 



Bunsen's photometer is very well adapted for prov- 

 ing the law that the relative intensities of two sources 

 of light are in the ratio of the squares of their distances 

 from the two surfaces which they respectively illuminate, 

 when the illumination of both surfaces is the same. For 

 the purpose of this proof, a single candle is rsed as a 

 source of light on one side of the screen, and a combi- 



