194 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
again subjected to the acid vapour, the colour was reproduced. How often this al- 
ternation might have gone on I cannot say, the specimen having been mislaid or 
destroyed. But a portion of such paper photographically impressed with a stamped 
pattern, accompanies this communication for the satisfaction of any Member who 
may wish to try the experiment. The extreme slowness of the action precludes any 
prismatic analysis of the process, and it cannot be too often repeated that the use of 
coloured glasses in such inquiries serves only to mislead. Of dormant photographic 
impressions generally, whether slowly developing themselves by lapse of time, or at 
once revivable by stimuli, as well as of the spontaneous fading and disappearance of 
such impressions, I shall have more to say hereafter, having encountered several very 
curious cases of the kind in studying the habitudes of gold, platina, &c. I would 
here only observe, that a consideration of many such phenomena has led me to regard 
it as not impossible that the retina itself may be photographically impressible by 
strong lights, and that some at least of the phenomena of visual spectra and secondary 
colours may arise from the sensorial perception of actual changes in progress in the 
physical state of that organ itself, subsequent to the cessation of the direct stimulant. 
Turmeric. — Further proofs of the continuation of the visible Prismatic Spectrum 
beyond the extreme Violet. 
185. The action of light on paper coloured with the alcoholic tincture of turmeric is 
but feeble. If long continued, however, it is whitened in the region of the blue and vio- 
let rays, from + 10 to -f 43, or thereabouts, the maximum being at -j- 23’5. The paper 
browned by carbonate of soda is somewhat more sensitive, especially when wet, in 
which case an abruptly terminated action is perceptible in the red region, giving rise 
to a double maximum at — 10‘0 and + 22'5, with an intermediate minimum at — 4'0. 
I should not have thought it necessary, however, to mention this paper, but on ac- 
count of a remarkable peculiarity in its reflective power, in virtue of which it renders 
very plainly visible a prolongation of the spectrum beyond the extreme violet, in the 
region of what I have termed in my last paper, the Lavender rays. As the experi- 
ment is easily made, and affords a ready method of rendering visible this part of the 
spectrum, I shall describe, with some minuteness, the appearances which presented 
themselves in my experiments, and which seem to place the real existence of those 
heretofore undescribed luminous rays beyond all reasonable objection, should any 
doubt have arisen as to the interpretation of the phenomenon described in my former 
paper (Art. 59.). 
186. Paper stained with tincture of turmeric is of a brilliant yellow colour, and in 
consequence, the spectrum thrown on it, if exposed in the open daylight, is considerably 
affected in its apparent colours, the blue portion appearing violet, and the violet very 
pale and faint ; but beyond the region occupied by the violet rays is distinctly to be 
seen a faint prolongation of the spectrum, terminated laterally, like the rest of it, by 
strait and sharp outlines, and which in this case affects the eye with the sensation of 
