1851 Great Exhibition: Official Catalogue: Class X.: Thomas Edge
702 EDGE, THOMAS, Great Peter Street, Westminster — Manufacturer.
Photometer; to ascertain the illuminating power of gas as compared with any other description of light; it is compact and portable, the disc is fixed at a certain distance from the standard light. Diagram to show the instrument in use.
[It is difficult to determine with accuracy the comparative intensity of light. If we suppose the quantity of light falling on a body to be the same as would have fallen on the place occupied by its shadow, it follows that the intensity of light decreases as the square of the distance increases, and therefore the denseness of the shadow of an interposing body becomes a means of determining the comparative intensities of light; for, if they appear of unequal density, it is sufficient to move the luminous bodies to those distances till the densities of the shadows be equal; then the intensities of the light or luminous bodies will be to each other as the squares of their respective distances from the interposing body.
Instruments for the purpose of determining the intensity of one light with another; of one strong light with several smaller lights; the comparative intensities of the light of the moon, and of that of a candle; the light of the heavens by night and by day, etc., have been invented at different times— J. G.]