SECT, xxiv.] INTENSITY OF CHEMICAL ACTION. 229 



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 beyond the extreme violet and 

 lavender rays, and in the other down to the extremest red : 

 with tartrate of silver the darkening occupies not only all the 

 space under the most refrangible rays, but reaches much be- 

 yond the extreme red. On paper prepared with formoben- 

 zoate 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 mer- 

 cury 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 refran- 

 gible 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 pre- 

 paration 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 refrangible 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 



