RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 
19 
(a beautiful crown-glass prism by Fraunhofer), on a large crown lens, and receiving 
the focus on paper prepared as in Art. 31. The result was equally striking and un- 
expected. A very intense photographic impression of the spectrum was rapidly formed, 
which, when withdrawn and viewed in moderate daylight, was found to be coloured 
with sombre, but unequivocal tints, imitating those of the spectrum itself. These 
tints were as stated in column 2 of the preceding Table, the colours at the correspond- 
ing points of the luminous spectrum being those in column 1. 
I have not succeeded in fixing these tints. They are, however, susceptible of half- 
fixing by the mere action of water, and may be viewed at leisure in moderate day- 
light, or by candlelight. And what is not a little remarkable, instead of fading, by 
keeping in the dark, they become much more evident and decided after a few days. 
And this, I would observe once for all, is a phenomenon of constant occurrence, what- 
ever be the preparation of the paper used, i. e. when colours are produced at all. 
55. The metallic yellow, induced by the blue and least refrangible violet rays, 
seems to mark the extreme action of the solar light on this particular preparation of 
paper, and perhaps on one or two other sorts, by the formation of a pellicle of metallic 
silver thick enough to reflect light with some degree of copiousness. Accordingly, 
this pellicle is apt to appear as a bright border or framing round the photographic 
copies of engravings, where not covered by the original, and when long exposed, and, 
in some cases, even in the strong lights of the pictures themselves. 
Extension of the visible Prismatic Spectrum. — A new Prismatic Colour. 
56. A variation in the experiment of Art. 54. deserves to be described, not so much 
on account of its bringing into view any photographic novelty, as for its confirming 
and placing in distinct evidence a fact which I have long more than suspected, but 
have always hitherto hesitated to announce. The fact to which I allude is that of 
the existence of luminous rays beyond the violet, but not affecting the eye with a sen- 
sation of violet, nor of any other of the recognised prismatic hues, but rather with 
that colour which is commonly termed lavender-grey; at least such is the impression 
i. e. to cobalt. This glass insulates two red rays of definite refrangibilities ; one which I call the mean red, the 
other the extreme red. This latter is situated so decidedly at the extremity of the visible spectrum, that if a 
dot be made in the centre of the well-defined and round solar image to which it corresponds, and the glass be 
then laid aside, that dot is judged by the eye to lie exactly at the end, or if anything rather beyond than within 
the end of the visible spectrum. This is the ray which forms the subject of a communication to the Royal 
Society by Mr. Cooper (read May 16, 1839), but which I have pointed out to the especial notice of photo- 
logists on several occasions, viz. 1st, in my paper on the action of crystallized media on homogeneous light, 
read December 23, 1819, and printed in the Philosophical Transactions for 1820 (See vol. cx. p. 88); 2ndly, in 
a paper read November 18, 1822, before the Royal Society of Edinburgh, and printed in the Transactions of 
that Society for 1822; 3rdly, in my Treatise on Light (Encyclop. Metropol. §§ 487, 497, 503.). This ray 
has, in fact, been constantly used and recommended by me as a standard ray, for which purpose it is admirably 
adapted on account of the facility of insulating it in a state of purity. 
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