Page:Collected Physical Papers.djvu/240

220 to light the plate was gently warmed for about 5 minutes over a spirit lamp, and then developed with mercury. The cut-out initials came out distinctly in dark lines. It would thus appear that since the plate was uniformly warmed, the difference between the screened and unscreened portions could only be in the different rates of emission.

The fourth stage in the curve for the action of light will be found specially interesting with reference to photographic reversals, which are found to be recurrent. Thus, starting with a neutral condition, we obtain the first negative with a moderate exposure; longer exposure will tend to reduce the intensity of the negative and give rise to a neutral condition. Further exposure gives rise to a positive, then a second neutral, and again a succeeding negative, and this may be repeated in recurrent series.

Such recurrent reversals are also exhibited (see fig. 49) by a substance under continuous mechanical vibration. In my paper on "Electric Touch" I have given similar instances of reversals produced by the action of long-continued electric radiation.

If molecular strain be the basis of all photographic phenomena, then it ought to be possible to obtain latent images by other methods of producing molecular strain.