On Lines in the Infra-red Region of the Solar Spectrum. 301 



Beyond these lines in the prismatic spectrum we have two 

 other pairs of bands far away down, and approaching the 

 theoretical limit of the least-refrangible end of the solar spec- 

 trum. The existence of these bands has, I believe, been sur- 

 mised by Becquerel by observations with the thermopile, 

 though I cannot at present find any published account of his 

 experiments on this point. 



The second point to which I wish to call attention is in 

 regard to the cause of the reversal of photographic image. 

 In a recent number of the Philosophical Magazine I gave a 

 detailed account of some experiments made in connexion with 

 the subject, showing that oxidation was the cause of most 

 reversals, though in some instances it might be produced by a 

 secondary action. The instance which Dr. Draper quotes 

 (where Claudet destroyed a daguerrean image by red light, 

 and was enabled to use the plate again) as showing that 

 oxidation was not the cause of reversal, is in reality corrobo- 

 rative of my observations. In the i Photographic Journal' for 

 1879 I have shown that where an image is destroyed by oxi- 

 dation, whether by an oxidizing agent or by the action of 

 light of low refrangibility, the plate is ready for a fresh ex- 

 posure to the image formed in the camera; and I believe it 

 is a common practice since that time for photographers to de- 

 stroy fog (or the effect of light on a plate) by using an oxidi- 

 zing solution. I may remark that the amount of oxidized 

 matter is as small as that forming (what is called) the latent 

 image itself, and is totally undistinguishable by any chemical 

 analysis ; and therefore it is not surprising that Claudet failed 

 to find any alteration chemically in the sensitive film. I am 

 not quite sure if I understand Dr. Draper to mean that W.L. 

 10,750 is beyond the theoretical limit of the spectrum. If 

 he does, I think he will see that it is in reality when the 



"W.L. is infinitely large — that is, when ^2=0, according to 

 Cauchy. X 



The plan of using phosphorescent material for viewing the 

 infra-red rays I have frequently tried with success, and can 

 quite confirm Dr. Draper's remarks. Mr. Warnerke was first 

 in the field with these observations ; and I left it to him to 

 communicate them, which he has neglected to do. The use of 

 phosphorescent material for any accurate measures is at present 

 out of the question, owing to the explanation which the emi- 

 nent American philosopher gives. 



Yours faithfully, 



W. DE W. AlBKEY. 



