measuring the comparative Intensities of Light. 71 
so great is the fluctuation in the quantity of light emitted by 
burning bodies, lamps, or candles, in all cases, even under the 
most favourable circumstances, that this is the source of the 
greatest difficulties I have met with in determining the rela- 
tive intensities of lights by the method here proposed. 
To ascertain by this method the comparative densities, or in- 
tensities of the light of the moon, and of that of a candle, the 
moon's direct rays must be received upon a plane white sur- 
face, at an angle of incidence of about 6o°, and the candle 
placed in the line of the reflection of the moon's rays from this 
surface ; when the shadows of the cylinder corresponding to 
the moon's light, and to that of the candle, being brought to 
be of equal density, by removing the candle farther oft; or 
bringing it nearer to the centre of the white plane, as the oc- 
casion may require, the intensity of the moon's light will be 
equal to that of the candle at the given distance of the candle 
from the plane. 
To ascertain the intensity of the light of the heavens by 
day or by night, this light must be let into a darkened room 
through a long tube, blackened on the inside, when its inten - 
sity may be compared with that of a candle or lamp by the 
method above described. 
To determine the intensity of the direct rays of the sun, 
compared to the light emitted by any of our artificial illumi- 
nators, it may perhaps be necessary, considering the almost in- 
conceivable intensity of the sun's light, to make use of some 
further contrivances and precautions, but I am convinced, 
however, that it may be done, and that even with a very con- 
siderable degree of precision. And when the relative inten- 
sity of the sun's light at the surface of the earth, compared 
