93 
measuring the comparative Intensities of Light. 
were placed over against each other before the photometer, 
each at the distance of 100 inches from the field of the instru- 
ment, and the light of B was brought to be of the same inten- 
sity as that of A-, or the shadows were brought to be of the same 
density, which was done by lengthening or shortening the 
wick of the lamp B, as the occasion required. This done, and 
the two lamps now burning with precisely the same degree of 
brilliancy, a pane of fine, clear, transparent, well polished 
glass, such as is commonly made use of in the construction of 
looking-glasses, six inches square, placed vertically upon a 
stand, in a small frame, was interposed before the lamp B, at 
the distance of about four feet from it, and in such a position 
that the light emitted by it was obliged to go perpendicularly 
through the middle of the pane, in order to arrive at the field 
of the photometer. The consequence of this was, that the 
light of the lamp B being diminished and weakened in its pas- 
sage through the glass, the illuminations of the shadows in the 
field of the photometer were no longer equal, the shadow cor- 
responding to the lamp A being now less enlightened by the 
light of the lamp B, than the shadow corresponding to the 
lamp B was enlightened by the undiminished light of the 
lamp A. 
To determine precisely the exact amount of this diminu- 
tion of the light of the lamp B (which was the main object of 
the experiment), nothing more was necessary 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 equa- 
lity of the shadows should be restored ; and this I found 
