light entering his room is likely to injure the sensitive surfaces there exposed; thus, having ascertained by experience that his plates are fogged, or his paper injured, when the revolutions exceed, say, ten a minute, he will take care to draw down an extra blind when the revolutions approach that number. Still more useful will the radiometer be in the photographic gallery. Placing an instrument near the sitter at the commencement of the day's operations, it is found that, to obtain a good negative, the lens must be uncovered—not for a particular number of seconds—but during the time required for the radiometer to make, say, twenty revolutions. For the remainder of the day, therefore, assuming his chemicals not to vary, the operator need not trouble himself about the variation of light; all he has to do is to watch the radiometer and expose for twenty revolutions, and his negatives will be of the same quality,[1] although at one time it may have taken five minutes, and at another not ten seconds, to perform the allotted number.
I have long been experimenting in the endeavor to trace some connection between the movements of attraction and repulsion above alluded to and the action of gravitation in Cavendish's celebrated experiment. The investigation is not sufficiently advanced to justify further details, but I will give here an outline of one of the results.
I find that a heavy metallic mass, when brought near a delicately-suspended light ball, attracts or repels it under the following circumstances:
I. When the ball is in air of ordinary density.
a. If the mass is colder than the ball, it repels the ball.
b. If the mass is hotter than the ball, it attracts the ball.
II. When the ball is in a vacuum.
a. If the mass is colder than the ball, it attracts the ball.
b. If the mass is hotter than the ball, it repels the ball.
The density of the medium surrounding the ball, the material of which the ball is made, and a very slight difference between the temperatures of the mass and the ball, exert so strong an influence over the attractive and repulsive force, and it has been so difficult for me to eliminate all interfering actions of temperature, electricity, etc., that I have not yet been able to get distinct evidence of an independent force (not being of the nature of heat or light) urging the ball and the mass together.
Experiment has, however, shown me that, while the action is in one direction in dense air, and in the opposite direction in a vacuum, there is (as I have already pointed out in the experiments described in the commencement of this paper) an intermediate pressure at which differences of temperature appear to exert little or no interfering ac-
- ↑ In this brief sketch I omit reference to the occasions in which the ultra-violet rays diminish in a greater proportion than the other rays.
