202 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
ing compound in respect of the oxides of iron, forming as it does Prussian blue with 
protosalts of that metal, but producing no precipitate with its persalts, indicate an 
excess of electro-negative energy, a disposition to part with oxygen, or, which is the 
same thing, to absorb hydrogen (in the presence of moisture), and thereby to return 
to its pristine state, under circumstances of moderate solicitation, such as the affinity 
of protoxide of iron (for instance) for an additional dose of oxygen, &c. 
203. Paper simply washed with a solution of this salt is highly sensitive to the ac- 
tion of light. Prussian blue is deposited (the base being necessarily supplied by the 
destruction of one portion of the acid, and the acid by the decomposition of another). 
After half an hour or an hour’s exposure to sunshine, a very beautiful negative 
photograph is the result, to fix which all that is necessary is to soak it in water, in 
which a little sulphate of soda is dissolved, to ensure the fixity of the Prussian blue 
deposited. While dry, the impression is dove-colour or lavender blue, which has a 
curious and striking effect on the greenish yellow ground of the paper produced by 
the saline solution. After washing, the ground colour disappears, and the photo- 
graph becomes bright blue on a white ground. If too long exposed it gets “ over- 
sunned,” and the tint has a brownish or yellowish tendency, which however is re- 
moved in fixing : but no increase of intensity beyond a certain point is obtained by 
continuance of exposure. 
204. Prismatic examination of this process demonstrates the remarkable and valu- 
able fact, that the decomposition of the salt and deposit of Prussian blue is due to 
the action of the blue and violet rays, the less refrangible rays below the blue having 
absolutely no influence either to exalt or diminish the effect. The limits of action 
are about + 18’0 and + 61 *0, fading insensibly both ways. The greatest intensity 
of action is at + 38. A feebler maximum occurs at + 23. The intensity of the 
impression is much increased by washing with acidulated water, still more if it hold 
in solution a little persalt of iron, but in this case the ground, if not very carefully 
defended from light, is blue. 
205. If a solution of this salt, mixed with perchloride of iron in a certain propor- 
tion, be washed over paper somewhat bibulous and exposed to the spectrum, a co- 
pious and intense deposit of Prussian blue takes place over the region indicated in 
the last article. But it does not terminate there. On the contrary, the action is con- 
tinued downwards in the spectrum, not only down to and beyond the extreme red 
rays, but far below, down to the very end of the thermic spectrum (as far as the spot 
called S in Art. 136, and even with some traces of the more remote spot a). The form- 
ation of the deposited colour in this region is accompanied with very singular pheno- 
mena, referable obviously to the heat developed by the thermic spectrum. Soon after 
the blue train, a h, fig. 9, in the positive region of the spectrum is formed, and has 
begun to acquire some intensity, an oval a, blunt at one extremity and pointed at the 
other, and of a dark brown colour, begins to appear. It enlarges rapidly, and at the 
same time throws forth a projection (3 , indicating the action of that portion of the 
