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XXIX. On Fixed Lines in the Ultra-red Region of the Spectrum. 

 By Captain Abney, E.E., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 (jrENTLEMEN, 



IN the February Number of the Philosophical Magazine 

 there is a communication from Dr. Draper in which refer- 

 ence is made to myself. May I ask that you will insert a few 

 remarks on the points in which I am interested ? 



May I premise by saying that I am excessively sorry if any 

 paper of mine has caused Dr. Draper to think that I have de- 

 preciated his method of photographing the least-refracted end 

 of the spectrum. I am very familiar with the method he has 

 indicated in his paper, and have employed it with marked suc- 

 cess. The object with which my experiments were undertaken 

 was to find some method by which the lines in the red and 

 ultra-red end of the spectrum could be photographed, in a 

 manner known as unreversed ; that is, that a black line in the 

 spectrum should, after being photographed by the collodion 

 process on glass, show as a transparent line in the picture 

 when viewed by transmitted light. It is fairly easy to obtain 

 a picture in which they appear opaque compared with the ad- 

 joining portions of the developed image; but it has hitherto 

 proved much more difficult to obtain them as indicated above. 

 in this attempt I have been fairly successful, and in some 

 negatives have obtained transparent lines far below those 

 already photographed. 



It may interest Dr. Draper to know that last autumn I 

 studied, with most encouraging results, the " antagonism " 

 which seems to exist between various portions of the spectrum 

 when they are used to excite a photographically sensitive plate. 

 This research I hope to continue when the sun appears a little 

 more frequently than it has during the last four months. 



I trust I may be allowed to differ from Dr. Draper regard- 

 ing the advisability of abandoning the researches previously 

 alluded to. It seems to me that a method of obtaining a nega- 

 tive picture of the least-refrangible portion of the spectrum will 

 be valuable in more ways than one. 



Yours faithfully, 



W. de W. Abney. 



