gg DETITHONIZING ACTION OF YELLOW SOLUTIONS. 



248. As we have said, the existence of these dark rays beyond the violet end of the 

 spectrum was established during the last century. For, when a solar spectrum is made 

 to fall on a piece of paper covered over with white chloride of silver, that compound 

 turns black, the colour changing where blue, indigo, and violet rays fall ; the change 

 also extending over those spaces which are outside of the violet ray, and where there 

 is no light. In 1836, by using bromide of silver instead of chloride, I found that these 

 deoxydizing rays extended out of the blue light down towards the yellow (Ap., 445) ; 

 these experiments were published in 1837. 



249. From a variety of evidence, obtained about the same time, it appeared that 

 when absorptive media were used, the rays of light could be deprived of much of their 

 chemical power. Thus, on passing a ray through a solution of bichromate of potash 

 (Ap., 410), it became almost inactive, refusing to produce any change on sensitive paper; 

 a similar result following on passing it through DALTON'S solution of the quadrosiilphu- 

 ret of lime, and a variety of other media (Ap., 410-509). At that early period in pho- 

 tographical investigations, when as yet no other chemist was engaged in these pursuits, 

 two important facts were published : 1st. The physical independence over a great part 

 of the spectrum of light and the tithonic rays (Ap., 386). 2d. That the tithonic rays 

 exhibit undoubted tokens of modifications answering to colours in light (Ap., 384, 433, 

 506). This is the foundation of the doctrine now more completely unfolded in this 

 work, under the designation of " The Theory of Ideal Coloration of the Tithonic Rays." 



250. From their constant association with light, it is difficult to give that clear evi- 

 dence of the physical independence of the tithonic rays which may be given in the 

 case of heat. Much weight may, nevertheless, be laid upon the circumstance that, by 

 prismatic action, these rays can be proved to exist beyond the extreme violet region, and 

 even beyond that ray which Sir J. HERSCHEL designates lavender. Evidence of the 

 same kind is derived from the partial action of absorbent solutions and media, which 

 act differently on each of these classes of rays. From observations of this kind tables 

 have been formed, with a view of proving this point (Ap., 673). In the Transactions 

 of the Royal Society, Sir JOHN HERSCHEL has given some of the same sort (Phil. 

 Trans., 1840). My results were published in 1842. 



251. At one time much stress was laid on the power which certain media, such as 

 bichromate of potash and quadrosulphuret of lime, possess in depriving a ray of its 

 chemical force. This argument is still resorted to by inaccurate writers, as affording 

 a very popular and palpable proof that light and the tithonic rays are different ex- 

 istences. " If," it is observed, " a ray of light is passed through a solution of quadro- 

 sulphuret of lime, and is then received on paper covered with chloride of silver, it is 

 found that the light alone has gone through, and the chemical rays have been absorbed. 

 The one, therefore, exists independent of the other." When I first observed the power 

 of these yellow solutions, this was the conclusion I came to (Ap., 410, 511). But the 

 theory of ideal coloration, presently to be explained, deprives this argument of much of 

 its force ; for the rays that are transmitted by such a solution are yellow rays of light, 

 and yellow tithonic rays ; in these the chloride of silver does not change, its decompo- 

 sition being brought about by the blue tithonic rays which have been absorbed along 



