88 Fixed Lines in the Ultra-red Region of the Spectrum. 



these spectrum-impressions were superb. If he will only try 

 the process, he will never give himself any concern about 

 collodion spectra again. 



I have attempted ineffectually to draw attention to this 

 process. There is, in my opinion, no fact more striking 

 among the chemical effects of light, none that promises, from 

 its investigation, more important results. 



There are two modes by which this process can be carried 

 into effect. 



1st. Submit a silvered plate to the vapour of iodine until 

 it has acquired a yellow tarnish ; or, better (since the plate 

 will become thirty times more sensitive), submit it to iodine, 

 bromine, and again to iodine, until the same tint of tarnish 

 has been obtained. 



. Now expose it to a pure spectrum in a room to which a 

 feeble daylight is admitted. On developing by the vapour of 

 mercury, a photograph will be obtained of the visible spectrum 

 from end to end, and extensive regions beyond the violet and 

 the red respectively. In all the part above the blue the day- 

 light and the sunlight have acted in unison, in all that below 

 the blue they have antagonized, and the plate remains unacted 

 upon, except where the Fraunhofer lines occur, and where, 

 therefore, there has been no sunlight. Then the daylight 

 has depicted those lines in white, while the more refrangible 

 are black. 



2nd. Prepare a plate as before. Expose it to a feeble day- 

 light or lamp- light, until, if developed with mercury, it would 

 whiten all over. But instead of developing it, now let it re- 

 ceive a pure spectrum. Then develop, and the result will be 

 the same as in the preceding case. 



So it is not necessary that the daylight and the sun- 

 light should act simultaneously; they may act successively 

 — an important fact in settling the nature of their anta- 

 gonism. 



To produce a perfect result, the two (the daylight and the 

 sunlight) must be exactly balanced. If the daylight should 

 preponderate, the protection is only in the extreme red ; as 

 it is diminished the protection extends higher and higher; 

 and the exact equipoise being attained, it reaches the confines 

 of the blue. All the Fraunhofer lines in the less-refrangible 

 portion of the spectrum come out in white ; all those in the 

 more refrangible are dark. In my early experiments I could 

 not obtain D, E, F; but my son, Henry Draper, operating 

 under this rule, has since photographed them all. 



Of all photographic facts, this antagonizing action is the 

 most extraordinary. I still work at its elucidation, though in 



