of the Solar Spectrum on Vegetable Colours. 173 
copious and richly coloured deposit of Prussian blue is formed 
over the whole of the blue, violet, and extra-spectral rays in 
that direction, extending downwards (with rapid graduation) 
almost to the yellow. If arrested when the blue is most in- 
tense and thrown into water, the impression is fixed, as in the 
accompanying specimen (see fig. 10.). But if the action of 
the light be continued, strange to say, the blue and violet 
rays begin to destroy their own work. A white oval makes 
its appearance in the most intense part of the blue (fig. 11.), 
which extends rapidly upwards and downwards. At a certain 
point of the action, the upper or more refrangible extremity 
of the white impression exhibits a semicircular termination, 
beyond which is a distinct and tolerably well-defined conju- 
gate image, or insulated circular white spot, whose centre is 
situated far beyond the extreme visible violet. 
207. If paper washed over with the mixed solution in ques- 
tion is exposed wet to sunshine, it darkens to a livid purple 
and rapidly whitens again. If the exposure be continued, 
the white again darkens gradually to a brownish violet hue. 
But in the shade it slowly resumes its original tint, after which 
it is again and again susceptible of the same round of action. 
The most singular and apparent capricious varieties of colora- 
tion and discoloration however arise (as is so frequently the 
case in photographic experiments) from different dosage of 
ingredients, order of washes, &c., so as to make the study of 
the phzenomena in a high degree complicated*. A certain 
adjustment of proportions gives an exquisite and highly sensi- 
tive positive photographic paper; another, a negative one, in 
which the impression of light, feeble at first, is strongly brought 
out afterwards by an additional wash of the ferrosesquicyanu- 
ret, &c. 
208. ‘The ordinary ferrocyanuret (the yellow salt), though 
not nearly so sensible to photographic action, is yet far from 
inert. In my former paper I have noticed its property of 
fixing against the further action of light, and ultimately de- 
stroying, photographic impressions on argentine papers. In 
conjunction also with preparations of silver, it has been made 
by Mr. Hunt the basis of a highly sensitive photographic 
paper. Its habitudes per se are, however, not a little remark- 
able. Paper simply washed with its fresh solution and ex- 
* The whitening is very obviously due to the deoxidation of the preci- 
pitated Prussian blue and the formation of the protp-ferrocyanuret of iron ; 
the resumption of colour in the shade, to the re-oxidizement of this com- 
pound, which is well known to absorb oxygen from the air with avidity. 
Simple Prussian blue, however, is not whitened by the violet rays. Its 
state must be peculiar. (See Postscript.) 
