RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 43 
the most refracted extremity of the spectrum doubled back on itself. On the other 
hand, this spot is absorbed by cupro-sulphate of ammonia, which also attacks the 
green, as well as by a rose-coloured glass, which lets the blue rays pass in abundance, 
but destroys both the green and violet. 
112. In further illustration of the action of coloured media, I may mention here 
some very curious results which have been communicated to me by Mr. Hunt above 
mentioned, and which I have his permission to insert. They seem to indicate some 
peculiar action of the barytic salts, but the habitudes of the media employed with re- 
lation to the pure prismatic rays not being stated, it is impossible to refer them to 
their precise origin in the spectrum. 
“ A paper,” says Mr. Hunt, “ prepared by washing with muriate of barytes and ni- 
trate of silver, allowed to darken whilst wet in the sunshine to a chocolate colour, 
was placed under a frame containing a red, a yellow, a green, and a blue glass. 
After a week’s exposure to diffused light, it became bright red under the red glass, a 
dirty yellow under the yellow, a dark green under the green, and a light olive under 
the blue. 
“ The above paper, washed with a solution of a salt of iodine, is very sensitive to 
light, and gives a beautiful picture. A picture thus taken was placed beneath the 
above four glasses, and another beneath four flat bottles containing coloured fluids. 
In a few days, under the red glass and fluid, the picture became a dark blue, under 
the yellow a light blue, under the green it remained unchanged, whilst under the 
blue it became a rose red ; and now, after three weeks, this red is slowly changing 
to green. 
“ I took a copy of an engraving under the glasses with the same paper. There was 
no action beneath the red glass, not much beneath the yellow, but beneath the blue 
it was very perfect, and, strange to say, beneath the green glass every line was faith- 
fully copied ; but the dark parts of the engraving were the brightest of the copy, as if 
it had been taken on one of the white photogenic papers.” 
VII. Of the exalting and depressing Power exercised by certain Media, under peculiar 
circumstances of Exposure to Solar Light, on the Intensity of its Chemical Action. 
113. When thin post-paper, merely washed with nitrate of silver, without any pre- 
vious or subsequent application, is exposed to a clear sunshine, partly covered by and 
strongly pressed into contact with glass, and partly projecting beyond it, so as to be 
freely exposed to air, the darkening produced in a given time is very unequal in the 
two portions. That protected by the glass, contrary to what might have been ex- 
pected, is very much more affected than the part exposed ; more, indeed, in some in- 
stances, than would be produced by free exposure during three or four times the given 
time. When fixed by hyposulphite of soda, the difference is rendered yet more strik- 
ing, to an extent hardly credible without trial. 
114. This singular effect was observed at the very outset of my inquiries, and was 
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