208 
SIR J. F. W. HERSCHEL ON THE ACTION OF THE RAYS 
character is required. The simplest and most convenient is that prepared by Mr. 
Hunt with one wash of muriate of ammonia, two of nitrate of silver*, and exposure 
to sunshine. With such paper (obligingly furnished me by Mr. Hunt himself) I 
made the following experiments. 
215. Exposed to the spectrum and washed with a solution of hydriodate of potash 
too weak fully to excite it-f'-, two contrary actions were produced by the rays above 
and below the zero point or mean yellow. By the former the paper began to be 
bleached at a point distant + 26’5 parts from the zero, from which point the bleach- 
ing extended gradually upwards to a considerable distance, and downwards to the 
circumference of a semicircle, having that point for a centre. By the latter the paper 
was darkened (at least in comparison with its general surface, which, purposely sub- 
jected to dispersed light, had begun to lose much of its original intense blackness), 
the darkness spreading also upwards and downwards : upwards till it passed the zero 
point, and nearly or quite attained the semicircle above mentioned ; and downwards 
to about — 19, or — 20 parts. As the paper dried the action seemed to be suspended. 
It became therefore necessary to renew the hydriodic wash, and thereby to increase 
the actual quantity of that salt present on the paper. Both actions grew more in- 
tense, but the bleaching effect most so. A perfect semicircle and long cometic train, 
c, d, fig. 12, No. 1, was produced, within which space the blackness of the paper was 
totally destroyed, and replaced by white or rather very pale yellow. The hydriodic 
washes being again and again renewed, the darkness at first produced in the lower 
part of the spectrum began to give way, and was slowly replaced by a very feeble 
bleaching, which at length extended very far indeed below the extreme red rays, 
and upwards to join the semicircle C fig. 12, No. 2, which had by this time assumed an 
outline perfectly sharp and well-defined, having its centre on the original point + 265 
of its commencement. But within this semicircle and its train, remarkable changes 
were observed to be all the while in progress. First, a somewhat dark, and greyish 
or brownish, perfectly circular and well-defined solar image arose, its diameter being 
somewhat less than that of the semicircular terminations, so as to leave a clear and 
distinct white border all around it, as represented by the dotted line in fig. 12, No. 2. 
Shortly after the complete formation of this spot, i. e. after its circular outline could 
be distinctly traced all round, it began to extend itself upwards into an oval or tailed 
form, but preserving its circular shape below and maintaining the white border invio- 
late, assuming at the same time a brownish yellow colour which gradually deepened, 
but never became intense. At the same time a very remarkable change was 
observed to take place in the reflective (or absorbent) powers of the paper in this re- 
gion. The violet-coloured end of the spectrum, which hitherto had been distinctly 
seen as usual occupying the space from + 30 to -f- 40 - 6, became quite indiscern- 
ible, while on the other hand the blue rays adjoining became reflected with such co- 
* Muriate of ammonia forty grains, water four ounces ; nitrate of silver sixty grains, water one ounce. 
t Ioduret of potassium sixty grains, water one ounce. 
