luminous index to travel several inches, slowing repulsion. A piece of ice brought near causes the spot of light to travel as much in the opposite direction. In order to insure the luminous index coming accurately back to zero, extreme precautions must be taken to keep all extraneous radiation from acting on the torsion-balance. The whole apparatus is closely packed round with a layer of cotton-wool about six inches thick, and outside this is arranged a double row of Winchester quart-bottles, filled with water, spaces only being left for the radiation to fall on the balance and for the index ray of light to get to and from the mirror.
However much the results may vary when the vacuum is imperfect, with an apparatus of this kind they always agree among them-selves when the residual gas is reduced to the minimum possible; and it is of no consequence what this residual gas is. Thus, starting with the apparatus full of various vapors and gases, such as air, carbonic acid, water, iodine, hydrogen, ammonia, etc., there is not found, at the highest rarefaction, any difference in the results which can be traced to the residual gas. A hydrogen vacuum appears the same as a water or an iodine vacuum.
The neutral point for a thin surface of pith being low, and that for a moderately thick piece of platinum being high, it follows that at a rarefaction intermediate between these two points pith will be repelled, and that platinum will be attracted by the same beam of radiation. This has been proved experimentally. An apparatus showing simultaneous attraction and repulsion by the same ray of light is illustrated in Fig. 6.
The pieces f g on the end of one beam consist of platinum-foil exposing a square centimetre of surface, while the extremities f' g' on the other beam consist of pith-plates of the same size. A wide beam of radiation thrown in the centre of the tube on to the plates g f' causes g to be attracted and f' to be repelled, as shown by the