British Association for the Adoancement of Science. 111 
Spectrum, which I have been led to notice in 1 the prosecution of 
my inquiries into the action of the spectrum on paper, rendered 
Sensitive to the chemical rays by Mr. Talbot’s process, or by 
others of my own devising. 
The property in question is this:—that the extreme red rays, 
(such, I mean, as are insulated from the rest of the spectrum by 
a dark blue plac colored by cobalt, and which are not seen in the’ — 
Spectrum unless the eye be dalenhed by such a glass from the 
glare of the other colors,) not only have no tendency to darken 
the prepared paper, but actually exert a contrary influence, and 
preserve the whiteness of paper on which they are received, 
when exposed at the same time to the action of a dispersed light 
sufficient of itself to produce a considerable impression. I have 
long suspected this to be the case, from phenomena observed in 
taking photographic copies of engravings; but having at length 
obtained demonstrative evidence of the fact, I think this may 
hot be an improper opportunity to announce it. 
When a slip of sensitive paper is exposed to a highly concen- 
trated spectrum, a picture of it is rapidly impressed on the paper, 
hot merely in back, but in colors, a fact which I ascertained 
nearly two months ago, and which observation of mine seems to 
have been alluded to (though in terms somewhat equivocal) by. 
M. Arago, in his account of Daguerre’s process. In order to un- 
derstand what follows, it will be necessary to describe the colors 
so depicted. The red is tolerably vivid, but is rather of a brick 
color than of a pure prismatic red ; and what is remarkable, its 
termination falls materially short of ‘the visible termination of the. * 
“Spectrum. The green is of a sombre, metallic hue ; the blue 
still more so, and rapidly passing into blackness. The yellow is 
deficient. The whole length of the chemical spectrum is not 
far short of double that of the luminous one, and at its more re- 
frangible end a slight ruddy or pinkish hue begins to appear. ‘The 
place of the extreme red, however, is marked by no color, thus ~ 
justifying so far the expression which M. Arago is reported to 
» have used in speaking of my experiments, “Le rayon rouge est 
Seul sans action.” 
It is impossible in this climate to form a brilliant and condensed 
Spectrum without a good deal of dispersed light in its confines ; 
~~. and this light, if the exposure of the paper be prolonged, acts, of 
. Course, on every part of its surface.- The colored picture in fornied, 
a? 
* 
