Comparing The Strength of Different Illuminants
It is not at all an easy matter to say that one kind of light is two, three, four, or more times as strong as another. We can make a rough guess and say that this illuminant is more powerful than that; but it is difficult to go further, if depending merely upon the eye without using any special apparatus.
Here is an easy method of comparing the strengths of various lights with great accuracy. Stand a white card vertically on the table and an inch or two in front of it stand a rod. A pencil will serve admirably if it does not insist on toppling over. Now get the lights which are to be compared in order to find out their relative strengths. Two will be sufficient for a first experiment.
Put one of the lights, say a candle, a foot from the screen, on the left side of the pencil. It throws a shadow of the pencil on the right side. Next arrange the other light, say an electric torch, on the right side of the pencil, also a foot away from the screen. In this case the shadow is on left side. Now, if the two shadows are of equal intensity, the two illuminants are of equal brilliancy, because they are both the same distance from the screen. In all probability, however, the shadow made by the torch will be much stronger than the other. If this is so, draw the torch backwards in the same straight line, until the shadow it casts is just as distinct and no more than that thrown by the candle. Now, measure the distances. The candle, we know, is one foot away, and we will suppose that the torch is three feet away.
This shows that the torch is nine times as powerful as the candle, because the intensity of the lights is inversely proportional to the square of their distances.