A TREATISE ON GAS-LIGHT.

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all directions from a luminous body, remainsundiminished in all distances from the centreof divergency. Thus we must suppose, thatthe quantity of light falling on every body, isthe same as would have fallen on the placesoccupied by the shadow; and if there wereany doubt of the truth of the supposition, itmight be confirmed by some simple experi-ment. Therefore, it follows, that, since theshadow of a square inch of any surfaces occu-pies at twice the distance of the surface fromthe luminous point the space of four squareinches, the intensity of the light diminishesas the square of the distance increases. If,consequently, we remove two sources of lightto such distances from an object that they mayilluminate it in equal degrees, we may con-clude that their original intensities are in-versely as the squares of the distances.

Hence, if two lights of unequal illuminatingpowers shine upon the same surface at equalobliquities, and an opake body be interposedbetween them and the illuminated surface,the two shadows produced, must differ inblackness or intensity in the same degree.For the shadow formed by intercepting the