gg LAWS DEDUCED FROM IT. 



containing a solution of ammonio-citrate of iron, of such strength, and in a stratum of 

 such thickness, as to appear of a bright yellow colour, I dispersed it by the prism, and 

 received the spectrum, as before, on chrysotype paper. For a long time the paper re- 

 mained unchanged, but after an hour's exposure I was able to bring out a very faint 

 mark, the position of which was 6, e,Jlg. 126, x. 



339. The inference which plainly arises from these experiments is, 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 ammonio-citrate of 

 iron, because they have been absorbed, and are already expended in effecting the re- 

 quired decomposition. 



340. The same conclusion was arrived at by experimenting in the following way: 

 I prepared a sensitive plate by exposure to iodine and bromine successively, which 

 gives, as is well known, a very changeable surface. This plate may be called, for the 

 sake of distinction, a test-plate. 



341. On this test-plate I received a spectrum formed from a beam which had passed 

 through the trough containing ammonio-citrate of iron. After a suitable exposure, I 

 found a stain reaching from j to o, fig. 126, y. But, as is shown in jig. 126, x, the 

 rays which affect the ammonio-citrate of iron reach from a to ft. Consequently, we 

 perceive that those which affect the test-plate are complementary to those which affect 

 the chrysotype. We draw, therefore, these farther conclusions : 



1st. That the rays which escape absorption by the ammonio-citrate of iron are pre- 

 cisely those which do not affect it chemically. 



2d. That the rays which are absorbed by tlie ammonio-citrate of iron are the rays 

 which produce chemical changes in it. 



342. Let us take a second case, selecting for consideration the bichromate of potash. 

 CASE OF THE BICHROMATE OF POTASH. As is well known, a piece of paper dipped 



in a solution of this salt speedily turns brown on exposure to the sun's rays ; but if there 

 be placed before it a trough containing a solution of the salt, then the change goes on 

 very slowly. 



343. Prismatic Analysis of this Case. On projecting a motionless prismatic spectrum 

 on this paper, an impression was obtained in a quarter of an hour, which extended from 

 a to p,Jig. 127, x. 



i 344. A trough with parallel faces, filled with a solution of the salt, was next inter- 

 posed in the beam, and the resulting spectrum received on a bromoiodized test-plate. It 

 extended from y to a, fig. 127, y. 



345. In reference to the test-plate used in these cases, its applicability depends on 

 a fact pointed out by Sir J. HERSCHF.L (Phil. Trans., 1840, p. 38), that bromide of sil- 

 ver is equally affected by all the rays of the spectrum. In using it as here described, 

 the experimenter must assure himself that sufficient of the bromine has been employed 

 to give sensitiveness to the extreme rays of the spectrum ; it should produce such a ti- 

 thouograph as that given in^/fg. 128, the red region being fully brought out. 



346. On examiuing^gs. 126, .r, 126, y, we perceive that they prove for the bichro- 



