144 REPORT—1850. 
seemed to be suspended over all that portion on which the less refrangible fell, 
and the shades of green and sombre blue which the latter would have im- 
pressed on white paper, were produced on that portion, which, but for their 
action, would have been merely blackened.” 
It was, however, subsequently proved by the same excellent experimental- 
ist, that if a paper, blackening under the influence of the red rays of the spec- 
trum, was repeatedly drenched with the solution of the iodine salts, the black- 
ening eventually gave way, and Was succeeded by a very feeble degree of 
bleaching. This bleaching appears to be distinctly due to thermic action, as 
it can be produced to an equal degree by the influence of heat alone. The 
action of the spectrum on this variety of paper may be divided into four 
arts s— 
ὃ Ist, bleaching by the most refrangible rays; 2ndly, blackening by the least 
refrangible rays ; then, 8rdly, bleaching by the same rays of low refrangibility ; 
and, 4thly, an actual darkening to a pale brown, by the more active chemical 
rays. 
If paper blackened by exposure is washed with a solution of iodide of po- 
tassium and exposed to sunshine, it is rapidly bleached. This forms the basis 
of a process for obtaining positive pictures which appears to have been noticed 
nearly about the same time by Dr. Fyfe, Lassaigne and myself, Dr. Fyfe, I be- 
lieve, being the first to publish his process, If, however, this paper is exposed 
to the prismatic spectrum, it will, at the same time as it is bleaching under 
the influence of the most refrangible rays, blacken under that of the least re- 
frangible; the iodine under the action of the chemical (actinic) rays com- 
bining and forming an iodide of silver, while under the operation of the calo- 
rific rays and others associated with them, an actual exaltation of the oxida- 
tion of the silver salt results. 
Here we have evidence of two sets of rays of widely different refrangibility, 
and consequently of dissimilar lengths of undulation, producing equally ener- 
getic chemical changes, but of an opposite character. ‘This might have been 
predicated by what we already knew of the action of the red and blue ends of 
the spectrum; but the experiment mentioned by Sir John Herschel, in which, 
under the combined influence of these two sets of rays acting upon one spot, 
an effect was produced which did not belong to either of them when separated, 
could not have been expected, and has not been explained upon any of the 
theories of light. 
A very interesting modification of the above phenomena may be produced 
by the use of coloured media. If an engraving is placed upon a piece of 
darkened photographic paper washed over with a solution of iodide of potas- 
sium, and it is then exposed to sunshine, a positive copy of the engraving, as 
has been already explained, results. Now, if we place a piece of blue glass 
over one portion, and a ruby glass over another, the bleaching process goes 
on with great energy under the blue, and the blackening with equal intensity 
under the red; and we obtain a positive and negative copy of the engraving 
at the same time on the same piece of paper. 
It has been proved by the experiments of Sir J. Herschel that this black- 
ening power is exerted by rays beyond the extreme red ray, where no lumi- 
nous influence can be detected. This result is particularly shown upon papers 
prepared with acetate of lead, chloride of platinum, and washed when under 
the influence of the light with hydriodate of potash. 
When experimenting with photographic papers prepared with the tartrate 
of potash and soda (Rochelle salts), Sir John Herschel observed that a pro- 
tected line presented itself on every side of the spectrum. ‘If the light was 
