8o LIFE AND WORK OF SIR JAGADIS C. BOSE 



radiation. The effects of recovery from moderate strain, 

 and of overstrain beyond speedy recovery, were also 

 noted. 



Hence at length the interesting paper ' On the 

 Strain Theory of Photographic Action/ 1 which, despite 

 its technical detail, is in principle intelligible enough even 

 to the non-photographer. The photographic effect in a 

 sensitive plate is demonstrated by its ' development ' 

 after exposure. This effect of light on sensitive substances 

 may be fugitive or persistent, with gradations between. 

 Bose's idea is that the image, with its lights and shadows, 

 produced differential strains on the sensitive matter 

 of the plate ; and that these differently light-strained 

 particles are consequently unequally attacked and fixed 

 by the developer. But if this image be correctly inter- 

 preted in terms of molecular strain, gradual recovery is to 

 be expected, with a subsequent fading of the image. The 

 early photographers, with their daguerreotypes, were much 

 troubled by this : hence subsequent photographic progress 

 has largely been through making plates of more enduring 

 quality. So that nowadays one goes on taking a series 

 of plates and films to be developed at leisure. Such 

 improved plates, on Bose's theory, simply delay or impede 

 the molecular elastic recovery of the variously strained 

 particles, which constitute the image, and hence give ample 

 time for its development. The term ' Sensitiser ' may 

 in many cases be a misnomer, since it may actually 

 cause a retardation of recovery. 



But the time of recovery should have its limit, and 

 it is here interesting to note that experience confirms 

 this. After Bose's exposition of his theory at the 

 Royal Photographic Society, one of the audience told 

 how after a photographic tour in India the development 

 of a batch of plates had been delayed by circumstances 

 for two years. On then proceeding to develop, he found 

 no image at -all : and this he had till then thought of as a 



1 Proc. Roy. Soc., 1901. 



