4 Experiments on the Relative Intensities 



case, considering the sheet of paper as a plane specu- 

 lum, the one light will be precisely in the line of reflec- 

 tion of the other. 



This may be easily performed, by actually placing a 

 piece of a looking-glass, 6 or 8 inches square, flat upon 

 the paper, in the middle of it, and observing by means 

 of it the real lines of reflection of the lights from that 

 plane, removing it afterwards as soon as the lights are 

 properly arranged. 



When this is done, a small cylinder of wood, about 

 \ of an inch in diameter and 6 inches long, must be 

 held in a vertical position about 2 or 3 inches before 

 the centre of the sheet of paper, and in such a manner 

 that the two shadows of the cylinder, corresponding to 

 the two lights, may be distinctly seen upon the paper. 



If these shadows should be found to be of unequal 

 dejtsitieSy which will almost always be the case, then 

 that light whose corresponding shadow is the densest 

 must be removed farther off, or the other must be 

 brought nearer to the paper, till the densities of the 

 shadows appear to be exactly equal, — or, in other 

 words, till the densities of the rays from the two lights 

 are equal at the surface of the paper ; when, the dis- 

 tances of the lights from the centre of the paper 

 being measured, the squares of those distances will be 

 to each other as the real intensities of the lights in 

 question at their sources. 



If, for example, the weaker light being placed at the 

 distance of 4 feet from the centre of the paper, it should 

 be found necessary, in order that the shadows may be 

 of the same density, to remove the stronger light to 

 the distance of 8 feet from that centre, in that case the 

 real intensity of the stronger light will be to that of the 



