30 Experiments on the Relative Intensities 



the same degree of brilliancy; all that is necessary 

 being to bring the shadows to be of the same density 

 with the glass and without it, noting the distance of 

 the lamp B in each case (the lamp A remaining im- 

 movable in its place) ; for the relative quantity of light 

 lost will ever be accurately shown by the ratio of the 

 squares of those distances, whatever be the relative 

 brilliancy with which the two lamps burn. The ex- 

 periment is more striking, and the consequences drawn 

 from it rather more obvious, when the lamps are made 

 to burn with equal flames ; otherwise that equality is of 

 no real advantage. 



Of the Loss of Light in its Reflection from the Surface 

 of a plane Glass Mirror. 



In these experiments the method of proceeding was 

 much the same as in those just mentioned. The 

 lamps A and B burning with clear, bright, and steady 

 flames were placed before the field of the photometer, 

 and one of them was moved backwards and forwards 

 till the illuminations of the shadows in the field of the 

 instrument were found to be precisely equal. 'The 

 distance of the lamp B being then noted, this lamp 

 was removed ; and a mirror being put in its place, but 

 nearer the field of the photometer, the lamp was so 

 placed that its rays, striking the centre of the mirror, 

 were reflected against the field of the photometer, 

 where, by bringing the lamp nearer to or removing it 

 farther from the mirror, the illumination of the field by 

 those reflected rays was now brought to be in equilib- 

 rium with the illumination of the standard lamp, and 

 then the distance of the lamp from the centre of the 



