1 86 Management of Light in Illumination. 



the horizontal line drawn on the field of the photome- 

 ter may be brought to be all in the same plane, which 

 is all that is necessary in order to the shadows being 

 brought to occupy their proper places. 



When this operation is finished (which may be per- 

 formed in a moment), the shadows must be brought to 

 be of the same density. This may be done either by 

 removing the stronger light farther off, or by bringing 

 that which is the most feeble nearer to the photometer. 



As the two shadows are reciprocally illuminated by 

 the two lights, it is perfectly evident that the shadow 

 which is least illuminated, or of the darkest shade, 

 must belong to the feeblest light, provided the light be 

 at the same distance from the field of the photometer ; 

 but, as the intensity of the light emitted by luminous 

 bodies decreases as the distance from the source of 

 that light increases, on removing the stronger light to 

 a greater distance the intensity of its illumination at 

 the field of the photometer will be diminished, and 

 the two shadows may be brought to be of the same 

 density. 



In that case it is quite certain that the intensity 

 of the light at the field of the photometer cannot be 

 greater on one side than on the other ; and, in order to 

 ascertain the relative intensities of the light emitted by 

 the flames of these candles, we have only to compare 

 the distances of those flames from the centre of that 

 field ; for those intensities must necessarily be as the 

 squares of those distances, which is a fact too well 

 known to require any elucidation. 



Instead of the rods divided into inches and tenths 

 of inches which I formerly used for measuring these 

 distances, I now employ flat rulers divided into degrees. 



