204 
SIR J. F. W. HERSCHEIj ON THE ACTION OF THE RAYS 
of proportions gives an exquisite and highly sensitive 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 ferrosesquicyanuret, &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 destroying, pho- 
tographic 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 remarkable. Paper simply 
washed with its fresh solution and exposed to the spectrum, slowly receives a deposit 
of Prussian blue over the region of the blue, violet, and £ ‘ lavender” rays : but this 
never becomes intense ; another series of changes commencing, indicated by the 
formation of a violet-coloured streak within the blue, just where the violet itself is 
most intense in the spectrum. If the solution be very feebly acidulated with sul- 
phuric acid, the first portion only of the spectral impression (from -f- 13*3 to + 20‘0) 
is blue, the whole of the remainder (extending to -J- 51) snulf brown. The dose of 
acid being increased, the exposure prolonged, and the liquid plentifully supplied, a 
green thermic impression is produced by the less refrangible rays, in which the 
spots a, (3, y are very distinct, and lie exactly (by measure) in their proper places. 
This impression continues as far as the zero point, where it begins to pass into blue, 
and graduates insensibly into the photographic spectrum, which attains its maximum 
of blue at -f- 25, and is thence prolonged onwards as a dull bluish streak on a brown 
ground, somewhat broader than itself, and projecting like a border on both sides. 
209. If paper be washed with a solution of ammonio-citrate of iron and dried, and 
then a wash passed over it of the yellow ferrocyanuret of potassium, there is no im- 
mediate formation of true Prussian blue, but the paper rapidly acquires a violet pur- 
ple colour, which deepens after a few minutes, as it dries, to almost absolute black- 
ness. In this state it is a positive photographic paper of high sensibility, and gives 
pictures of great depth and sharpness, but with this peculiarity, that they darken 
again spontaneously on exposure to air in darkness, and are soon obliterated. 
The paper, however, remains susceptible to light and capable of receiving other pic- 
tures, which in their turn fade, without any possibility (so far as I can see) of arrest- 
ing them ; which is to be regretted, as they are very beautiful, and the paper of such 
easy preparation. If washed with ammonia or its carbonate, they are for a few mo- 
ments entirely obliterated, but presently reappear, with reversed lights and shades. In 
this state they are fixed, and the ammonia, with all that it will dissolve, being re- 
moved by washing in water, their colour becomes a pure Prussian blue, which deepens 
much by keeping. If the solutions be mixed there results a very dark violet-coloured 
of the proto-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.) 
