Ultra-red Invisible Region of the Spectrum. 87 



beyond the line A, and had designated them as a, ft, 7. Of 

 the existence of a fourth, still lower down, I had obtained 

 imperfect evidence. 



Three years subsequently these lines were rediscovered 

 by MM. Foucault and Fizeau, who used the photographic 

 method previously discovered by me. In 1871 they were 

 again detected by M. Lamansky by the aid of a thermo- 

 multiplier. 



I formerly supposed that the experiments of Sir John 

 Herschel, made with paper blackened on one side and washed 

 with alcohol on the other, indicated the existence of these 

 lines ; but a more attentive consideration of the apparatus he 

 employed has led me to change that opinion. He did not 

 use a slit, but the direct image of the sun, which with the 

 optical train he had was more than a quarter of an inch in 

 diameter. Under such circumstances it was impossible that 

 either these or any other lines could be seen. The result he 

 obtained was a succession of circular patches or spots — solar 

 images — commencing above the yellow, and continuing into 

 the ultra-red. 



More recently Captain Abney has experimented in the 

 same direction, using collodion containing various colouring 

 or other material supposed to promote the photographic action 

 of the less-refrangible rays ; and in a very recent Number of 

 Poggendorff's Annalen (No. 10, 1876), MM. H. C. Vogel 

 and 0. Lohse have published similar experiments. It is this 

 last paper that leads me to make the present remarks ; for those 

 physicists seem not to be aware that what they are attempting 

 now was accomplished in America thirty-five years ago. 



I think, from some expressions that Captain Abney has 

 used in one of his papers, that he entertains a very low 

 estimate of the photographs so produced ; he depreciates the 

 process by which they were obtained very much. Sir John 

 Herschel, than whom no one was a more competent judge 

 of a fine photograph, says of one of these that I sent him 

 (Philosophical Magazine, February 1843), " 1 should hardly 

 be doing justice to the beauty of the specimen itself as a joint 

 work of nature and art were I to forbear acknowledging its 



arrival, and offering a few remarks on it The spectrum 



itself is extremely remarkable and beautiful Want of 



habitude in the manipulation of the daguerreotype process, and 

 by no means want of sun, prevented my obtaining any thing 

 like so fine an impression." If Captain Abney will for once 

 excuse an inventor for praising his own invention, I who 

 have seen very many photographs, and know the difference 

 between a good and an imperfect one, will assure him that 



