174 ‘Sir J. F. W. Herschel on the Action of the Rays 
posed 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 in- 
tense in the spectrum. If the solutigm be very feebly acidu- 
lated with sulphuric acid, the first portion only of the spectral 
impression (from + 13°3 to + 20°0)is blue, the whole of the 
remainder (extending to + 51) snuff 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, 6, 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 
+ 25, and is thence prolonged onwards as a dull bluish streak 
on a brown ground, somewhat broader than itself, and pro- 
jecting 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 immediate formation 
of true Prussian blue, but the paper rapidly acquires a violet 
purple colour, which deepens after a few minutes, as it dries, 
to almost absolute blackness. 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 sus- 
ceptible to light and capable of receiving other pictures, which 
in their turn fade, without any possibility (so far as I can see) 
of arresting 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 moments 
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 removed by washing in 
water, their colour becomes a pure Prussian blue, which 
deepens much by keeping. Ifthe solutions be mixed there 
results a very dark violet-coloured 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 ques- 
tion, which is most sensitive when wet. 
210. It seems at first sight natural to refer these curious 
and complex changes to the instability of the cyanic com- 
pounds, and that this opinion is to a certain extent correct, is 
