Cotmt Rumford’s Account of a Method of 
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 shewn that the intensities 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 100 2 to 90, a 2 , or as 1 to ,8136 ; so that no more 
than ,8136 parts of the light which impinged against the 
glass found its way through it ; the other ,186'q 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 ex- 
periment, I now removed the pane of glass, and found that the 
equality of the shadows was again restored, when the lamp B 
arrived at its former station, 100 inches from the field of the 
photometer. 
This experiment I repeated no less than 10 times, and 
found the loss of light in its passage through this pane of 
glass, taking a mean of all the experiments, to be ,1973 parts 
of the whole quantity that impinged against it ; the variations 
in the results of the various experiments being from ,1720 to 
In four experiments, with another pane of the same kind of 
glass, the loss of light was ,1836 ; ,1732 ; ,2056 ; and ,1853 , 
mean ,1869. 
When the two panes of this glass were placed before the 
lamp B, at the same time, but without touching each other. 
