SCIENTIFIC SUMMARY. 
257 
ments, M. Claudet exposed a sensitive daguerreotype plate to light, and then 
printing on it from a negative covered with a yellow glass, obtained by 
development a positive image, by which it seems that the action of white 
light is capable of being destroyed by that of yellow light. If this were not 
the case, such an image would not have been so obtained on a plate of this 
kind. Printed on from a negative in the ordinary way without the preliminary 
exposure to light, and the after printing by yellow light, the daguerreotype 
plate would, as might be expected, give a negative image. 
The Latent Image. — During the past quarter a considerable amount of 
study has been devoted to the latent image formed by light on iodide of 
silver. The questions associated with this inquiry were the first to which 
scientific attention was devoted when photography was new, and we have yet 
to learn anything positive in connection therewith. This invisible impression, 
so subtle and refined in its nature as to defy our efforts to define it, yet 
sufficiently strong not to be destroyed by powerful chemical action, is held by 
some to be merely physical, and by others to be chemical. Some controversy 
on this subject has appeared in the photographic journals, inaugurated in the 
British Journal of Photography by Mr. Carey Lea, who says, “ Every chemist 
has had occasion to remark that with some slowly-formed precipitates there 
is a tendency -to deposit on all the portions of the vessel which have been 
touched by a rod used in stirring. This is no rare or unusual case, but one 
of very common occurrence. The mere light drawing of a glass rod over a 
glass surface alters it in some most curious way, so that the saline precipitate 
seeks out every line with the most exact discrimination, and pours upon it a 
white thread-like deposit ; and if the mixture has been much stirred previous 
to the formation of the precipitate, so that the edges of the rod have 
travelled over many parts of the glass, the most intricate and interwoven lines 
will be developed and rendered visible by the formation of the precipitate.” 
Mr. Lea goes on to say, “ If now we add sulphate of iron to a solution of 
nitrate of silver, metallic silver is thrown down, but not instantly ; and in 
this condition of things the invisible image attracts the silver out of its old 
solvent, now by chemical changes no longer capable of holding it in solution, 
but just in the same way as the invisible lines of the rod did in the case above 
referred to.” Mr. Hardwick, in his “ Manual of Photographic Chemistry,” 
took the same view of this question many years ago, and gave the same 
experiments as Mr. Lea describes, with others of a cognate description. Of 
course we have nothing conclusively demonstrated by these analogies. 
M. Poiteven, followed by Dr. Vogel, insists that the action of light upon 
iodide of silver is purely chemical, but their arguments seem to us no more 
conclusive on the other side, and there are many phenomena connected with 
development which their theories do not explain. How can solarization, for 
instance, be satisfactorily accounted for by the chemical theory? If this 
theory, as they explain it, be correct, it must follow that the longer the action 
of the light the more powerfully impressed would be the image, whereas it is 
well known that the reverse is the case — at least where a developer is 
employed. 
Is Pure Iodide of Silver sensitive to Light ? — This question has been com- 
monly answered in the negative, and most of the great authorities have been 
opposed to the idea of its sensitiveness to light. But Mr. Carey Lea, by an 
