376 Mr. A. Claudet on the principal Ph(enomena of 



In a paper I communicated to the Royal Society on the 17th 

 of June 184-7 (see Transactions), and an abstract of which I 

 read before the Association at Oxford, 1 stated that the red, 

 orange and yellow rays were destroying the action of white 

 light, and that the surface was recovering its former sensitive- 

 ness or unaffected state after having been submitted to the 

 action of these rays. I inferred from that curious fact that 

 light could not have decomposed the surface ; for if it had, it 

 would be difficult to understand how the red, orange, or yel- 

 low rays could combine again, one with another, elements so 

 volatile as bromine and iodine, alter they had been once se- 

 parated from the silver. 



But I had not yet been able to ascertain that, when light has 

 decomposed the bromo-iodide of silver, the red, orange or 

 yellow rays cannot restore the surface to its former state. The 

 action of light, which can be destroyed by the red, orange or 

 yellow rays, does not determine the decomposition, which would 

 require an intensity 3000 times greater. It is the kind of action 

 produced by an intensity 3000 times less, giving the affi- 

 nity for mercury, which is completely destroyed by the red, 

 orange or yellow rays. It seems, therefore, that I was right 

 in saying that there was no decomposition of the compound 

 during the short action which is sufficient to give the affinity 

 for mercury, and in ascribing the formation of the image only 

 to that affinity. White light, or the chemical rays which 

 accompany it, communicate to the surface the affinity for mer- 

 cury, and the red, orange, or yellow rays withdraw it. 1 

 must notice here a singular anomaly j viz. that when the 

 sensitive surface is prepared only with iodine without bromine, 

 the-red, orange or yellow rays, instead of destroying the action 

 of white light, continue the effect of decomposition as well as 

 that of affinity for mercury. Still there is a double compound 

 of iodine which is far more sensitive than the simple com- 

 pound, and on which the red, orange, or yellow rays exercise 

 their destructive action as in the case of the bromo-iodide. 



The phaenomenon of the continuing action of the red, 

 orange or yellow rays, on the simple compound of iodide of 

 silver, was discovered by M. Ed. Becquerel ; and soon after 

 M. Gaudin fountl, that not only those rays continue the action 

 by which mercury is deposited, but that they develope without 

 mercury an image having the same appearance as that pro- 

 duced by mercurial vapour. 



M. Gaudin, not having observed the fact of the white pre- 

 cipitate, which is the result of the decomposition by the action 

 of light, could not explain the cause of the image brought out 

 under the influence of the yellow ray. 



