4f70 Prof. Draper on the Interference Spectrum, 



Case of the Chrysotype. — The sensitive material em- 

 ployed in this process is the ammonio-citrate of iron, a solu- 

 tion, which, when viewed through a small thickness, is of a 

 yellow colour. 



Prismatic analysis of the Chrysotype. — I projected a motion- 

 less spectrum on chrysotype paper, and speedily obtained an 

 impression of a pale brown colour, which, when brought out 

 by neutral chloride of gold, was found to extend from about 

 the fixed line F, to beyond the violet extremity. 



Having passed a beam from a heliostat through a trough 

 with parallel sides, containing a solution of the ammonio-ci- 

 trate of such a strength, and in a stratum of such 9. thickness, 

 as to appear of a bright yellow colour, I received its spectrum 

 on chrysotype paper. For a long time the paper remained 

 unchanged, but after an hour's exposure I was able to bring 

 out a very faint mark, the centre of which was nearly upon 

 the line F, and the stain extended a little above and below 

 that line. 



On considering these two results, it will appear that the 

 active chrysotype rays are absorbed by the ammonio-citrate 

 of iron ; or in other words, that this substance is sensitive, 

 because it absorbs a peculiar class of rays. No change can 

 take place in chrysotype paper by rays that have passed 

 through a sufficient thickness of ammonio-citrate of iron, be- 

 cause the active ones have been absorbed, and are already ex- 

 pended in effecting the required decomposition. 



The same conclusion was arrived at by experimenting in 

 the following way : — I prepared a sensitive plate by exposure 

 to iodine and bromine successively. This gives, as is well 

 known, a very changeable surface. Such a plate may be called, 

 for the sake of distinction, a test-plate. 



On this test-plate I received a spectrum formed from a 

 beam which had passed through the trough containing am- 

 monio-citrate of iron. After a suitable exposure I found a 

 tithonograph extending from the extreme red slightly beyond 

 the fixed line F. But we have already seen that the active 

 chrysotype rays extend from about F to the end of the violet. 

 Consequently, those rays which affect the test-plate after ab- 

 sorption are complementary to those which affect the chryso- 

 type. The following conclusions may therefore be drawn : — 



1st. That the rays which escape absorption by the ammo- 

 nio-citrate of iron are precisely those which do not affect it 

 chemically. 



2nd. That the rays which are absorbed by the ammonio- 

 citrate of iron are the rays which produce chemical changes 

 in it. 



