ON THE PHOTOGRAPHIC IMAGE. 103 



to germinate and take on growth in the Australian family, the uniformity of 

 cranial character now prevailing would be concomitantly and progressively 

 modified. It is certain that such modifications of cranial structure and feature, 

 accompanying diversities in modes of life, detract from their value as 

 distinctive natural-history characters of races of mankind. 



Supposing social progress to be possible in a race like the Australians, 

 without admixture of other blood, a question of much interest suggests 

 itself — in what degree and in what way the cranial physiognomy would be 

 modified ? By analogy I think it probable that the modification might, in 

 the course of time, become at least as great as that which is observable in 

 unmixed Negro races which for generations have been subjected to, and im- 

 proved by, civilizing influences. 



Upon the whole, then, in regard to the immediate subject of the present 

 Report, undertaken at the request of the Committee of the Ethnological 

 Section, and performed on that account, as well as out of regard for my 

 accomplished and scientific friend Mr. Hodgson, with much pleasure and the 

 best of my leisure and ability, I must confess that the results are rather nega- 

 tive than positive; but if they should suggest any improved views in the 

 study and application of the physical characters of Man, the aim of the 

 Section will not wholly have been unfulfilled. 



Report of the Committee, consisting of Messrs. Maskelyne, Hadow, 

 Hardwich, and Llewelyn, on the Present State of our Knowledge 

 regarding the Photographic Image, 



The chemical problem presented by the photographic image is one of great 

 complexity. It is uninviting to the chemist in so far as it presents very 

 little opportunity of his obtaining quantitative results ; for howsoever subtle 

 and rapid be the chemical transformation effected by the light, it consists, in 

 most cases, of a superficial change only, and defies even the delicate methods 

 of the balance. In undertaking to collect what is known and to test the 

 correctness of what has been published regarding this intricate problem, the 

 Committee have proposed to themselves to deal first with the simplest trans- 

 formations on which photographic processes are founded, and to pass on 

 from these to the more complex. 



Moreover they confine themselves to the photographic results obtained 

 with the salts of silver, as these are the most employed, and because it is 

 necessary to assign some limits to their inquiry. 



If the salts of silver are the most remarkable for their susceptibility to 

 photochemical change, one is naturally led to search first for the causes of 

 this among those simpler compounds of the metal in which the transforma- 

 tion is not complicated by the secondary decompositions which might be 

 expected to accompany it in the case of organic compounds. Yet among 

 the inorganic compounds this susceptibility to photochemical decomposition 

 is rare ; and though not absolutely confined to one salt, the chloride of silver, 

 that body exhibits the simplest and one of the best illustrations of it. 



The chloride of silver, when perfectly pure, passes, on exposure to light, 

 from its pure white through various stages of change in hue, in which blue 

 is mixed with grey, until it finally reaches a deep slate-violet colour. Chlo- 

 rine is evolved from the chloride ; but the question which here meets us in 

 limine is one which probably underlies the whole of the problem we have to 

 consider, and consists in the chemical condition in which the silver remains 



