28 Experiments on the Relative Intensities 



tometer. The consequence of this was that, the light 

 of the lamp B being diminished and weakened in its 

 passage through the glass, the illuminations of the 

 shadows in the field of the photometer were no longer 

 equal, the shadow corresponding to the lamp A bein^ 

 now less enlightened by the light of the lamp B than 

 the shadow corresponding to the lamp B was enlight- 

 ened by the undiminished light of the lamp A. 



To determine precisely the exact amount of this 

 diminution of the light of the lamp B (which was the 

 main object of the experiment), nothing more was nec- 

 essary than to bring this lamp nearer to the field of 

 the photometer, till its light passing through the glass 

 should be in equilibrium with the direct light of the 

 lamp A, or, in other words, till the equality of the 

 shadows should be restored ; and this I found actually 

 happened when the lamp B from 100 inches was 

 brought to the distance of 90.2 inches from the field 

 of the photometer. 



Now, as it has already been shown that the intensi- 

 ties of the lights are as the squares of their distances 

 from the field of the photometer, the illuminations being 

 equal at that field, it is evident that the light of the 

 lamp B was diminished, in this experiment, in its 

 passage through the pane of glass, in the ratio of 

 ioo a to 9O.2 2 , or as i to .8136; so that no more than 

 .8136 parts of the light which impinged against the 

 glass found its way through it, the other .1864 parts 

 being dispersed and lost. 



To assure myself that the lamps still continued to 

 emit the same relative quantities of light as at the 

 beginning of the experiment, I now removed the pane 

 of glass, and found that the equality of the shadows 



