measuring the comparative Intensities of Light. 83 
brought to be of the same intensity at the surface of the ver- 
tical plane, were really stronger at one time than at another, 
the equal shadows of the cylinders would be proportionally 
deeper, and that by comparing, at different times, the density 
of those shadows with a painted scale of shades, regularly gra- 
duated, any difference in. the intensity of the standard light 
might be discovered and compensated ; but upon making the 
experiment I found, what indeed a little patient reflection 
would have enabled me to foresee, that the apparent den- 
sity of the two equal shadows corresponding to the lights 
compared with a painted scale of shades, exposed in the same 
light, is ever the same* however the intensity of the rays at the 
surface upon which those shadows are projected may vary. 
There is however another method by which I think it pro- 
bable that the standard lamp may be adjusted with the requi- 
site degree of precision. It appears, from a considerable 
number of experiments, of which I shall hereafter give a more 
particular account, that the quantity of light emitted by a 
lamp which burns in the same manner, with a clear flame, and 
without smoke , is in all cases as the quantity of oil consumed. 
If, therefore, the standard lamp be so adjusted as always to 
consume a certain given quantity of oil in a given time, there 
is much reason to suppose that it may then be depended on as 
a just standard of light. 
In order to abridge the calculations necessary in these in- 
quiries, it will always be advantageous to place the standard 
lamp at the distance of 100 inches of the photometer, and to 
assume the intensity of its light at its source equal to unity ; in 
this case (calling this standard light A, the intensity of the 
light at its source = x = 1 ; and the distance of the lamp from 
M 2 
