OF THE SOLAR SPECTRUM ON VEGETABLE COLOURS. 
205 
ink, which may be kept uninjured in an opake bottle, and will readily furnish, by a 
single wash, at a moment’s notice, the positive paper in question, which is most sensi- 
tive when wet. 
210. It seems at first sight natural to refer these curious and complex changes to 
the instability of the cyanic compounds, and that this opinion is to a certain extent 
correct, is proved by the photographic impressions described in Arts. 204 and 209, 
where no iron is added beyond what exists in the ferrocyanic salts themselves. 
Nevertheless the following experiments abundantly prove that in several of the 
changes above described, the immediate action of the solar rays is not exerted on these 
salts, but on the iron contained in the ferruginous solution added to them, which it 
deoxidizes or otherwise alters, thereby presenting it to the ferrocyanic salts in such a 
form as to precipitate the acids in combination with the peroxide or protoxide of iron, 
as the case may be. To make this evident, all that is necessary is simply to leave out 
the ferrocyanate in the preparation of the paper, which thus becomes reduced to a 
simple washing over with the ammonio-citric solution. Paper so washed is of a bright 
yellow colour, and is apparently little, but in reality highly sensitive to photogra- 
phic action. Exposed to strong sunshine for some time indeed, its bright yellow tint is 
dulled into an ochrey hue, or even to gray, but the change altogether amounts to a 
moderate per centage of the total light reflected, and in short exposures is such as 
would easily escape notice. Nevertheless, if a slip of this paper be held for only four 
or five seconds in the sun (the effect of which is quite imperceptible to the eye), and 
when withdrawn into the shade be washed over with the ferrosesquicyanate of potash, 
a considerable deposit of Prussian blue takes place on the part sunned, and none 
whatever on the rest, so that on washing the whole with water, a pretty strong blue 
impression is left, demonstrating the reduction of iron in that portion of the paper to 
the state of protoxide. The effect in question is not, it should be observed, peculiar 
to the ammonio-citrate of iron. The ammonio- and potasso-tartrate fully possess, 
and the perchloride exactly neutralized partakes of the same property: but the expe- 
riment is far more neatly made and succeeds better with the other salts. 
211. If a long strip of paper, prepared as in the last article, be marked off into 
compartments and subjected to graduated exposure to sunshine, so that the times of 
exposure in each succession shall form an arithmetical progression of l m , 2 m , &c., 
and when withdrawn washed over as aforesaid with the ferrosesquicyanuret and 
rinsed in water, the blue deposit is found to increase with the time of exposure up to 
a very deep and full colour, after which its total intensity, so far from increasing, di- 
minishes, and at length almost vanishes. Again, if a slip of the same paper be ex- 
posed a long while to the spectrum, the whole impression consists in a feeble ochrey- 
brown streak, extending over the region of the blue, violet and lavender rays as far as 
about + 55. But on the application of the cyanic solution (in the shade) a most in- 
tense blue spectrum is developed over the whole of the more refrangible region, 
in the interior of which the blue colour appears to have been, as it were, eaten away. 
