Action of the Ilai/s of the Solar Spectrum. 333 



4. The chemical analysis of the solar spectrum forms the subject 

 of the next section of his jiaper. It has long been known that rays 

 of different colours and refrangibilities exert very different degrees 

 of energy in effecting chemical changes; and that those occupying the 

 violet end of the spectrum possess the greatest deoxidating powers. 

 But the author finds that these chemical energies are distributed 

 throughout the whole of the spectrum ; that they are not a mere 

 function of the refrangibility, but stand in relation to physical quali- 

 ties of another kind, both of the ray and of the analysing medium ; 

 and that this relation is by no means the same as the one which de- 

 termines the absorptive action of the medium on the colorific rays. 

 His experiments also show that there is a third set of relations con- 

 cerned in this action, and most materially influencing both the amount 

 and the character of the chemical action on each point of the spec- 

 trum ; namely, those depending on the physical qualities of the sub- 

 stance on which the rays are received, and whose changes indicate 

 and measure their action. 



The author endeavoured to detect the existence of inactive spaces 

 in the chemical spectrum, analogous to the dark lines in the lumi- 

 nous one ; but without any marked success. The attempt, however, 

 revealed several curious facts. The maximum of action on the most 

 ordinary description of photographic paper, namely, that prepared 

 with common salt, was found to be, not beyond the violet, but about 

 the confines of the blue and green, near the situation of the ray F in 

 Fraunhofer's scale : and the visible termination of the violet rays 

 nearly bisected the photographic image impressed on the paper : in 

 the visible violet rays there occurred a sort of minimum of action, 

 about one-third of the distance from Fraunhofer's ray H, towards G : 

 the whole of the red, up to about Fraunhofer's line C appears to be 

 inactive ; and lastly, the orange-red rays communicate to the paper 

 a brick-red tint passing into green and dark blue. Hence are de- 

 duced, first, the absolute necessity of perfect achromaticity in the 

 object-glass of a photographic camera ; and secondly, the possibility 

 of the future production of naturally coloured photographs. 



5. The extension of the visible prismatic spectrum beyond the 

 space ordinarily assigned to it, is stated as one of the results of these 

 researches ; the author having discovered that beyond the extreme 

 violet rays there exist luminous rays affecting the eyes with a sen- 

 sation, not of violet, or of any other of the recognised prismatic hues, 

 but of a colour which may be called lavender-yrey, and exerting a 

 powerful deoxidating action. 



6. Chemical properties of the red end of the spectrum. The rays 

 occupying this part of the spectrum were found to exert an action of 

 an opposite nature to that of the blue, violet, and lavender rays. 

 When the red rays act on prepared paper in conjunction with the 

 diffused light of the sky, the discolorating influence of the latter is 

 suspended, and the paper remains white; but if the j)aper has been 

 already discoloured by ordinary light, the red rays change its actual 

 colour to a bright red. 



7. The combined action of rays of different degrees of refrangibi- 



