198 INTENSITY OF CHEMICAL ACTION. SECT. XXIV. 



tremity. This distribution does not depend on the re- 

 frangibility of the rays alone, but also on the nature of 

 the rays themselves, and on the physical properties of 

 the analyzing medium on which the rays are received, 

 whose changes indicate and measure their action. The 

 length of the photographic image of the same solar spec- 

 trum varies with the physical qualities of the surface on 

 which it is impressed. When the solar spectrum is 

 received on paper prepared with bromide of silver, the 

 chemical spectrum, as indicated merely by the length of 

 the darkened part, includes within its limits the whole 

 luminous spectrum, extending in one direction far be- 

 yond the extreme violet and lavender rays, and in the 

 other down to the extremest red : with tartrate of sil- 

 ver the darkening occupies not only all the space under 

 the most refrangible rays, but reaches 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 car- 

 bonate of mercury it ends in the blue, and on paper 

 prepared with the per cyanide 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 in- 

 digo 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 tar- 

 trate 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 refrangible end of the 

 solar spectrum the greatest chemical power is exerted 



