Jagadish Chandra Bose

“ The above considerations afford a simple explanation of the very obscure phenomenon of the relapse of the invisible image. Recovery is merely a question of time. With certain substances it is immediate, with others it takes a little longer, as in a daguerreotype, where the latent image only disappears in the course of several hours. In ordinary photographic plates the recovery may not take place for several years. We have seen how the strain effect of electric radiation was transient in some cases, whereas it was persistent in others. ”

“ It has been shown that reversals are produced by intense radiation; we may thus have reversals of the first, second, and succeeding higher orders. Now it is possible that the reversal, or the dark-lightning effect, may be obtained, not only by a subsequent diffuse illumination (Clayden effect) , but also by the action of lightning itself, provided that the intensity of illumination is sufficiently great and sudden to produce the reversal. The luminous intensity of lightning discharge is incomparably higher than any that can be produced by a spark from an electric machine. ”

“ In the above case, it is difficult to see how light could have produced any chemical action in the relatively inactive silver salt. It is more likely that the effect produced was of the nature of some physical strain. That light does produce molecular strain even at such low temperatures—a strain which may remain latent—is shown from Professor Dewar's experiments on phosphorescence. Ammon. Pt. Cyanide cooled to - 180° C. in liquid air absorbs light, but emits feeble radiation. But as the temperature is raised the stored-up light is emitted with very great intensity. ”

“ Even a highly elastic substance may be rendered more or less permanently distorted by straining it beyond the limit of elastic recovery; or (2) the presence of a retarding substance may prevent the self-recovery of the sensitive material. One of the chief functions of the so-called sensitisers may be to prevent self-recovery and make the after-effect permanent.
4. Permanance of the After-effect by Overstrain.
Thus in many cases where images cannot be obtained with ordinary exposure, they can be obtained with excessive strain caused by prolonged exposure. ”

“ The first part of the curve, or the latent period, is very suggestive as regards the obscure phenomenon of photo-electric induction. Thus "Quantitative measurements have shown that the action of light is not instantaneous. On the contrary, it gradually develops, and requires a considerable time before it attains its full strength. When a mixture of chlorine and hydrogen, which have been kept in the dark, is exposed to the light, there is either no hydrochloric acid or only a very small quantity formed in the first moment ”

“ It seems to me that in this experiment, as the plate was uniformly warmed, the difference between the screened and unscreened portions could only be in the different rates of emission.
9. Phenomenon of Recurrent Reversals.
The fourth stage in the curve for the action of light (see fig. 16) will be found specially interesting with reference to photographic reversals. These reversals are found to be recurrent. Thus, starting with a neutral condition, we obtain the first negative with a moderate exposure ”

“ Thus Moser obtained an invisible image on a clean silver plate by exposing it to the sun for 2 hours or more. The invisible image was afterwards fixed by development with mercury vapour. A similar result was obtained with copper. ”