172 Sir J. F. W. Herschel on the Action of the Rays 
iron in a certain proportion, be washed over paper somewhat 
bibulous and exposed to the spectrum, a copious 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 continued 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 Sin Art. 136, and even with some traces of the 
more remote spot «). The formation of the deposited colour 
in this region is accompanied with very singular phenomena, 
referable obviously to the heat developed by the thermic spec- 
trum. Soon after the bluetrain, a 4, fig. 9, [Plate II.], 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 ap- 
pear. It enlarges rapidly, and at the same time throws forth a 
projection , indicating the action of that portion of the thermic 
spectrum so characterized in Art. 136. It also acquires a 
whitish narrow border, indicated by the dotted line, and very 
conspicuous on the green ground of the paper. ‘The action 
continuing, the spot y is marked out by the extension of the 
border in that direction, soon after which the spot appears, 
in brown. Lastly appears ¢ with feeble traces of further irre- 
gular and interrupted action. Measurements of these spots 
as they appear, leave no doubt of their identity in situation 
with the thermic spots a, B, y, 6 of Art. 136, and that they 
are referable to the drying of the paper is shown by the fact, 
that a film of the liquid dried in a porcelain saucer changes 
from green to dark brown at a definite point of dryness. 
Moreover, on wetting the paper, the brown spots disappear, 
and in their place we find a train of Prussian blue, of varying 
intensity, but of uniform breadth (not swelling and contracting, 
as is the case with the heat-spots formed by simple drying, 
and therefore obviously due to direct radiation), and termi- 
nating in two insulated and tolerably well-defined circular 
spots or solar images, holding precisely the places of y and 8 
(viz. at — 35°7 and — 45°1). 
206. If in lieu of the perchloride of iron, we substitute a 
solution of that curious salt the ammonio-citrate of iron, the 
photographic effects are among the most various and. remark- 
able that have yet offered themselves to our notice in this 
novel and fertile field of inquiry. The two solutions mix with- 
out causing any precipitate, and produce a liquid of a brown 
colour, which washed over paper is green (being strongly di- 
chromatic). If this be done under the prism, the action of 
the spectrum is almost instantaneous, and most intense. A 
