of the Solar Spectrum on Vegetable Colours. 17 
goes on rapidly. J¢ does not even cease in the dark when once 
begun. Hence it happens that photographic impressions 
taken on such paper, which when fresh are very sharp and 
beautiful, fade by keeping, visibly from day to day, however 
carefully preserved from light. Specimens of such photo- 
graphs (copies of engravings) are submitted with this paper 
for inspection. They require from half an hour to an hour to 
complete, according to the sunshine. Hydriodate of potash 
cautiously applied, retards considerably, but does not ulti- 
mately prevent, this spontaneous discharge. 
176. Exposed to the spectrum, in about fifteen or twenty 
minutes the colour is totally destroyed and the paper whitened 
in the whole region of the green, blue and violet rays, to which 
therefore the most energetic action is confined, agreeably to 
the law of complementary tints (Art. 170.). If the action of 
the spectrum be prolonged, a much feebler whitening becomes 
sensible in the red, and a trace of it also beyond the violet 
into the *“*Javender” rays. In this state the type of the im- 
pressed spectrum (in an experiment made on the 7th of April 
in the present year) was as in fig. 4, indicating three obsolete 
maxima c, d, e, and a very sudden diminution of the action 
at b, f, the dimensions being as follows: Ya = — 9:4, Y 4 
=+71, Ye = + 12°5,Yd= + 23°5, Ye = + 34:0, Yf 
= + 41:4, Yg = + 59°7.. The paper thus impressed was 
again re-examined on the 2nd of May, or after twenty-five days, 
during which interval it had been exposed to free air, but 
only to feeble and dispersed occasional lights. It was found 
to have undergone a remarkable change, two distinct white 
spots having become insulated, or nearly so, at the very ex~ 
tremities of the impressed spectrum, the three maxima above 
indicated having also become much more distinct, and two 
new, subordinate ones, having begun to show themselves in 
the faint traces connecting the spots above mentioned with the 
main impression. ‘The type of the spectrum in this state was 
as represented in fig. 5, and the places of the several maxima 
being as follows:—Ist, — 10:0; 2nd, — 0°5; 3rd, + 12:0; 
4th, +29:0; 5th, + 40°0; 6th, + 50::; 7th, + 61:0. The 
terminal spot at the red extremity was nearly equal in diameter 
to the sun’s image; that at the least refracted end, corre- 
sponding in place to rays much beyond the last violet, was 
smaller, but perfectly distinct, and as it constitutes the only 
instance I have yet encountered of a definite ray in this region 
of the spectrum*, 1 have been thus particular in describing 
the phanomenon. 
* Since this was written, other cases, extremely remarkable, among the 
argentine preparations, have presented themselves. Sce Art. 214. 
Phil. Mag. 8. 3. Vol. 22. No. 142, Jan. 1843. C 
