506 Sir J, F. W. Herschel on certain improvements 
have succeeded at last, by the simple addition of corrosive 
sublimate to the ammonio-citrate of iron with which the paper 
is prepared. ‘The improved process, therefore, may be thus 
stated. Mix together equal measures of a saturated cold so- 
lution of corrosive sublimate, and a solution of ammonio- 
_ citrate of iron, one part by weight of the salt to eleven parts 
water. No immediate precipitation takes place, and before 
any has time to do so, the mixture must be washed over paper 
(which should have rather a yellowish than a bluish cast), and 
dried. It is now ready for use, and I do not find that it is 
impaired by keeping. To use it, it must be exposed to the 
light till a faint, but yet perfectly visible picture is impressed, 
and till the border (if it be an engraving which is copied) has 
assumed a pale brown colour. Being withdrawn it is to be 
brushed over as rapidly as possible with a broad flat brush, 
dipped in a saturated solution of prussiate (ferrocyanate) of 
potash diluted with three times its bulk of gum-water, so 
strong as just to flow freely without adhesion to the lip of the 
vessel. All the care that is required is, that the film of liquid 
be very thinly, evenly, and above all, quickly spread. Being 
then allowed to dry in the dark, it rarely fails to produce a 
good picture. And what is very remarkable, it is zpso facto 
fixed as soon as dry, so at least as not to be injured by ex- 
posure to common day-light, immediately; and after a few 
days’ keeping it becomes entirely so, and will bear strong 
lights uninjured. By long keeping, details at first barely seen 
come out, and the whole picture acquires a continually-in- 
creasing intensity, without however sacrificing distinctness ; 
and by the same gradations its colour passes from purple to 
greenish-blue. Some experience, to be acquired only by 
practice, is necessary to determine the proper moment for 
withdrawing the photograph from the action of the light. If it 
be over-sunned, only the darker shades appear; if too little, 
the whole, though beautifully perfect in the first moments of 
its appearance, speedily runs into an indistinguishable blot. 
233. ‘The principal obstacle in the way of the employment 
of gold and silver as photographic ingredients for the pro- 
duction of negative models, to be used for retransfers, so as _ 
to multiply positive copies, arises from the want of absolute 
opacity in these metals or their oxides when in a state of mi- 
nute division, ‘The same objection does not apply, or applies 
with much less force, tomercury, which (probably owing to its 
fluid state, which prevents its particles from acquiring that 
excessive tenuity which a laminated form would admit, by 
reason of their capillary forces contracting each separately de- 
posited particle into a sphere) is one of the most opake sub- 
