of the Solar Spectrum on Vegetable Colours. 9 
that the picture penetrates the paper and appears on the back 
of very nearly the same intensity as on the face*. Indeed, if 
the picture be over-sunned the back will exhibit a perfect im- 
pression, while the face is spoiled, which produces a very 
strange effect: exposed to the spectrum, the blue colour is 
converted to a pale reddish yellow in the region of the less 
refrangible rays, and simply whitened in the more refrangible 
region. ‘The action, when prolonged till the light seems to 
have no further influence, extends from — 12:4, corrected for 
semidiameter, to +40, or thereabouts, where it dies away in- 
sensibly. ‘The maximum of photographic action occurs at 
—8°7, and some trace of a minimum is perceptible at + 11:5. 
Photographs taken on this paper, or spectra impressed on it, 
are fugitive—lose much of their force and beauty in a few days, 
and at length vanish altogether. 
155. When paper is washed with a solution of guaiacum 
in soda it acquires a green colour, though the solution itself is 
brown. By inclining the paper and carrying the wash al- 
ways from below upwards, a very even tint may be obtained. 
The excess of liquid being blotted off, aqueous solution of 
chlorine was poured over it (on a slope) till all the alkali was 
saturated, and the liquid ran off smelling strongly of chlorine. 
Thus was produced a paper (No. 1168.) very eyenly tinted, 
and varying in colour from a deep, somewhat greenish, to a fine 
celestial blue, according to the strength of the solutions em- 
ployed. It is very sensitive, and is attacked with especial 
energy by rays in the spectrum, ranging from —11:4 to-+ 11-4 
with a maximum at —9-0, the type being as in fig. 3. 
156. When paper so prepared is exposed, wet, to a tem- 
perature of 212° Fahr., it is immediately discoloured, the 
green changing to a sere or brownish yellow. The same 
change is produced after some little time at a temperature of 
190°, and still more slowly, though yet completely, at 180°. 
At 175° the discoloration is incomplete and very slow; and 
below that temperature the colour is not affected. If the paper 
be perfectly dried in a temperature gradually raised to 212°, 
the discoloration requires a considerably higher temperature, 
ranging from 220° to 275°, according to the time of exposure, 
being very slow at the former limit and almost immediate at 
the latter. ‘These changes are independent of the action of 
light, being produced under mercury. 
157. The destruction by heat of the green or blue colour 
superinduced on guaiacum by the more refrangible rays of 
light, was noticed by Wollaston, and it would seem, ‘on a 
* For another remarkable case of this kind see the Postscript to this 
paper, 
