214 SIR J. F. W. HERSCHEL ON THE ACTION OF THE SOLAR RAYS. * 
consistency of cream, is easily (and with certain precautions of manipulation*) very 
evenly spread on paper, producing - photographic tablets of every variety of sensibility 
and inertness, according to the proportion of the doses used. By combining all three 
of the ingredients, and adding a small quantity of tartaric acid^, a paper is produced 
of a pretty high degree of sensibility (more than by the use of either separately), 
which in about half an hour or an hour, according to the sun, affords pictures of such 
force and depth of colour, such velvety richness of material, and such perfection of 
detail and preservation of the relative intensities of the light, as infinitely to surpass 
any photographic production I have yet seen, and which indeed it seems impossible 
to go beyond. Most unfortunately, they cannot be preserved. Every attempt to fix 
them has resulted in the destruction of their beauty and force ; and even when kept 
from light, they fade with more or less rapidity, some disappearing almost entirely in 
three or four days, while others have resisted tolerably well for a fortnight, or even a 
month. It is to an over-dose of tartaric acid that their more rapid deterioration 
seems to be due, and of course it is important to keep down the proportion of this 
ingredient as low as possible. But without it I have never succeeded in producing 
that peculiar velvety aspect on which the charm of these pictures chiefly depends, 
nor anything like the same intensity of colour without over-sunning. 
230. I might here describe many other curious and interesting photographic results 
to which, under the genial influence of such a summer as, possibly, has never before 
been witnessed in England, I have been conducted. But in so doing I should surpass 
the reasonable bounds of a Postscript illustrative of my text, and abuse the privilege 
accorded me. Yet I cannot forbear noticing one at least, in which a line or dot 
engraving of any degree of delicacy is imitated, line for line, and dot for dot in a 
manner which might deceive any but a practised artist to the point of rendering him 
unable to declare that the photograph had not been struck off from the original plate 
with common printing ink, by the ordinary process of copper-plate printing. The 
details of this process, which are delicate and somewhat tedious, cannot properly be 
stated here ; if for no other reason, because I have not yet obtained a complete com- 
mand over the result : but a microscopic examination of the specimens placed in the 
hands of our worthy Secretary, though somewhat marred by the accidents of mani- 
pulation, will I think suffice to justify the terms employed above. 
* The cream should be spread as rapidly as possible over the whole paper, well worked in, cleared off as 
much as possible, and linished with a brush nearly dry, spread out broad and pressed to a strait thin edge, 
which must be drawn as lightly and evenly as possible over every part of the paper till the surface appears free 
from every streak, and barely moist. 
f One measure of a solution of ammonio-citrate, and one of a solution of ammonio-tartrate of iron, contain- 
ing, each, one-tenth of its weight of the respective salts. Tartaric acid, saturated solution, one-eighth of the 
joint volumes of the other solutions. Form a cream by pouring in as rapidly as possible one measure of a satu- 
rated solution of the proto-nitrate and well mixing with a brush. 
