RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 41 
yet more refracted rays, sufficient to destroy all photographic action over that part 
of the spectrum, and to confine its effective range between the limit of the green and 
blue rays on one hand, and the extreme violet on the other. 
108. A green glass limits the photographic spectrum to a short oval patch extend- 
ing from the fiducial point, or a very little below it, to -f 18 or 20, in which no co- 
loured fringe can be traced. The glass used was of a pure grass green hue of con- 
siderable intensity, neither verging to the yellow on the one hand nor to a blue on 
the other. When sulphate of nickel was used, the impression was of somewhat greater 
extent in both directions, viz. from about — 3 to + 28 ; but in both cases the action 
of the indigo and violet rays, and of all beyond them, was totally cut off. 
109. Among dichro'ite media (or those which in some certain thickness insulate 
two portions of the luminous spectrum, or which are much more eminently trans- 
parent for two rays than for all the rest,) there is none more remarkable than the 
muriate of chromium*, which reduces the spectrum to two narrow and pretty well 
defined spaces, coloured, the one red and the other green, the red being that at 
the extremity of the spectrum, and the green of great purity and richness of tint. 
Nor is the action of the spectrum thus analysed on photographic papers less remark- 
able. On such paper (prepared with common salt) it impresses, under a considerable 
thickness of the medium, two circular spots, the one intensely black, the other white, 
the places of whose centres coincide with those of the coloured images, exactly in the 
green, and nearly so in the red, only somewhat lower, indicating the action of invi- 
sible rays. When the liquid is more dilute, or the thickness diminished, so as to 
render the coloured images perceptibly oval, these spots, in like manner, extend into 
ovals, and moreover, a third, much fainter, makes its appearance in the region beyond 
the violet, where no perceptible illumination falls. As the liquid is more diluted, or 
its thickness more reduced, this spot gains strength, and connects itself by a faint 
train of darkness with the much more intense one corresponding to the green image. 
In like manner also may the cobalt-blue glass already so often spoken of, which is in 
effect a polychro’ite medium, be made to insulate a definite circular white spot in the 
photographic impression of its spectrum, corresponding to those extreme red rays 
which it insulates in such perfection. To return, however, to our chromic solution 
It is impossible not to be struck with the much greater energy of chemical action in 
the green which this medium insulates, than with that insulated by the green glass 
already alluded to ; and the following experiment presents us with a green of con- 
siderable illuminating power, which is almost or quite devoid of chemical action. 
* Vide article Light, Encyclopedia Metropolitana, under the head of Absorption, where the curious pro- 
perties of this medium were, I believe, first pointed out. At that time I regarded them as unique, for the di- 
chro'ite action of sap-green is not nearly so definite; but I have since found the same property in two other 
liquids, viz. in a sulphuric solution of sublimed indigo diluted with alcohol, and in a sort of ink sold under the 
name of Stephen’s Writing Fluid: the green in this last, however, is less pure and definite; and it differs 
also from the other two in extinguishing the red with more energy than the green, in consequence of which its 
ultimate tint is green, while that of the others is red. 
MDCCCXL. 
G 
