OF THE SOLAR SPECTRUM. 547 
reduced salt of silver, but the light also acts on the salt of silver 
already affected, as we have established for the salts of silver. If 
then the layer of iodide is weak, this second reaction may pre- 
vail over the first and blacken the paper in the least refrangible 
_ parts of the spectrum. 
In order to guard against this effect, when once the paper is 
_ affected, care must be taken to wash it with common water and 
then to plunge it into a solution of iodide of potassium strong 
_ enough to avoid these two reactions. The effect also depends on 
_ the manner in which the paper has been at first affected. If it 
has been strongly blackened, the action does not extend beyond 
the blue; but if it has been less black, it extends as far as the red. 
5 Fig. 6 represents the image of the spectrum obtained by pro- 
; jection on a sheet of paper prepared with the precautions before 
ae 
_ indicated, and by affecting the paper slightly before the experi- 
_ ment, an idea may be obtained of the distance to which the che- 
_ mical action extends. By means of this paper, all the lines which 
_ we have already described are depicted with tolerable clearness. 
__ By help of Mr. Talbot’s method, which consists in bringing 
F- gallic acid to act by means of heat on a paper done over with 
_ iodide of silver, and which has received the impression of the 
_ image intended to be produced, tolerable results are obtained ; 
but as the sensibility of this paper is very great, it is necessary 
that the action should be of as short duration as possible, if not 
the lines would not be distinct. This method gives an inverse 
image, that is to say, the shadows are represented by bright places, 
and vice versd. 
We have not only made use of papers prepared with salts of 
silver, but also of papers done over with other sensible substances, 
‘ such as the bichromate of potash, the resin of guaiacum, &c. 
We know that a sheet of paper prepared with bichromate of 
_ potash, when it is dry, changes its colour more rapidly in the light, 
_ and from a pale yellow becomes a dark wood colour. This colora- 
tion, which arises from the conversion of the chromic acid into 
| _ oxide of chrome through the action of organic substances under 
| ; the influence of light, has its maximum of action in the spectrum 
| at the limit of the green and the blue, or towards the line F. 
Fie 7 gives an idea of the action of the spectrum on this paper, 
it being understood that the lines are omitted. The action ceases 
; almost instantaneously at the extremity of the green, beyond the 
| lines F and 4; but on the most refrangible part of the spectrum 
} ° 
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