ANALYSIS OF THE CHRYSOTYPE. 85 



Tithonic red. 



Tithonic orange. 



Tithonic yellow, &c., &c., 



the expressions pointing out the region of the spectrum under consideration, but 

 drawing a strong and perfect distinction between the agent involved and light. In the 

 same way other derivates will arise, such as tithonic white or black. 



331. When a beam of light is dispersed by the action of a prism, and the resulting 

 spectrum examined by physical tests, it is easy to recognise that the quality of impart- 

 ing a sensation of difference of colour is not the only token of intrinsic difference in 

 the character of rays of different refrangibilities. Writers on the mechanical theory of 

 optics have assumed, that the constitutional distinction between the various colour 

 giving rays is the different lengths of waves which they represent. To a given index 

 of refrangibility there belongs a particular length of wave, and a particular tint of col- 

 our. But there are facts connected with the history of light which seem to prove that 

 beyond this there are peculiarities which are far more profound. 



332. Several years ago, Sir D. BREWSTER showed that, by resorting to absorbent 

 media, rays of any colour could be insulated in every part of the spectrum ; that red 

 light existed in the violet spaces, and blue light in the red. These results, being sub- 

 stantiated, would appear to afford a very formidable argument against the dependance 

 of colour on wave length. 



333. I do not propose to enter here on any speculative considerations respecting the 

 physical causes which enable light to impart to an organ of vision the phenomena of 

 tints, but to show that the idea of coloration must be admitted for the chemical rays. 



334. As, in the foregoing chapter, we gathered our final views respecting absorption 

 from considerations originally drawn from one case ; so, in this, let us examine the 

 phenomena exhibited in one or two cases, and then generalize from them. We have 

 already seen (Ap., 597), that when a ray has impinged on a sensitive surface, as on a 

 Daguerreotype plate, and been reflected by it, it has lost, to a great extent, the power 

 of again producing the same effect, or is detithonized. 



335. The remarkable process discovered by Sir JOHN HERSCHEL, and called by him 

 chrysotype, enables us to verify in an easy and very satisfactory way the truth of this 

 remark. 



336. CASE OF THE CHRYSOTYPE. The sensitive material employed is the ammonio- 

 citrate of iron; a solution, which, when viewed through small thicknesses, is of a yellow 

 colour. From its being a solution, it is peculiarly fitted for these experiments. 



337. When a piece of paper, washed over with this yellow solution, is exposed to 

 the sun behind a trough containing the same solution, the paper is found to change 

 very slowly, showing that the liquid in the trough is absorbing the active rays. 



338. Prismatic Analysis of the Chrysotype. (.) I projected a motionless 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 a to ft, Jig. 

 126, x. 



(b.~) Having passed a beam from the heliostat, through a trough with parallel sides, 



