208 SOLAR SPECTRUM. SECT. XXIV. 



most refrangible rays, but readies much beyond the extreme red. 

 On paper prepared with formobenzoate of silver the chemical 

 spectrum is cut off at the orange rays, with phosphate of silver 

 in the yellow, and with chloride of gold it terminates with the 

 green, with carbonate of mercury it ends in the blue, and on 

 paper prepared with the percyanide of gold, ammonia, and 

 nitrate of silver, the darkening lies entirely beyond the visible 

 spectrum at its most refrangible extremity, and is only half its 

 length, whereas in some cases chemical action occupies a space 

 more than twice the length of the luminous image. 



The point of maximum energy of chemical action varies as 

 much for different preparations as the scale of action. In the 

 greater number of cases the point of deepest blackening lies 

 about the lower edge of the indigo rays, though in no two cases 

 is it exactly the same, and in many substances it is widely 

 different. On paper prepared with the juice of the ten- week 

 stock (Mathiola annua) there are two maxima, one in the mean 

 yellow and a weaker in the violet ; and on a preparation of 

 tartrate of silver Sir John Herschel found three, one in the least 

 refrangible blue, one in the indigo, and a third beyond the visible 

 violet. The decrease in photographic energy is seldom perfectly 

 alike on both sides of the maximum. Thus at the most re- 

 frangible end of the solar spectrum the greatest chemical power 

 is exerted in most instances where there is least light and heat, 

 and even in the space where both sensibly cease. 



Not only the intensity but the kind of action is different in 

 the different points of the solar spectrum, as evidently appears 

 from the various colours that are frequently impressed on the 

 same analyzing surface, each ray having a tendency to impart 

 its own colour. Sir John Herschel obtained a coloured image 

 of the solar spectrum on paper prepared according to Mr. Talbot's 

 principle, from a sunbeam refracted by a glass prism and then 

 highly condensed by a lens. The photographic image was 

 rapidly formed and very intense, and, when withdrawn from 

 the spectrum and viewed in common daylight, it was found to 

 be coloured with sombre but unequivocal tints imitating the 

 prismatic colours, which varied gradually from red through 

 green and blue to a purplish black. After washing the surface 

 in water, the tints became more decided by being kept a few 

 days in the dark a phenomenon, Sir John observes, of constant 



