Distilled water . 7 oz. 7 drms. 2 .x ,, Potassium bromide 32'4 grs. 225 ccs.
B. Nelson's No. x gela tine 7715 grs. 5 8.
Silver nitrate (re cryst.) . lf 3 $) Distilled water 7 oz. 7 drms. 225 ccs. Bring each solution to 95° F. (35° C.), and add B to A with continual stirring.
Lippmann's later formula is as follows Potassium bromide . 8x grs. '53 g.
Gelatine . . . 62 4 Distilled water . . 3i oz, boo ccs.
For colour sensitising add : Alcoholic solution cyan ine (I : 500) . . 90 mins. 6 ccs.
Alcoholic solution chino line red (I : 5oo) . 45 3 y, Mix the above at 95° P. and add Silver nitrate (dry) . IIi grs. 75 and stir until dissolved. Filter through glass wool and coat the plates, allow to set, place each plate in alcohol, then wash for half an hour, drain, and dry. The plates will keep a long time.
Senior increases the general sensitiveness of the emulsion by adding to every 3i oz. or boo ccs., 3 grs. or 2 g. of silver eoside.
For fixing all plates a 15 per cent. solution of " hypo " may be used, or a 5 per cent, solution of potassium cyanide, but the latter must only be allowed to act for x5 to 20 seconds.
The heliochromes may be intensified with dilute mercuric chloride followed by amidol and sulphite, or the latter alone, and they may be reduced by a very weak " hypo " and ferricyanide bath.
One of the most exhaustive researches of the Lippmann process has been undertaken by H. E. Ives, whose conclusions will be found in the British Journal of Photography (1908).
As already stated, the sensitive film must be in contact with a reflecting surface such as mer cury, and special dark-slides can be obtained commercially for this process. These have usually a hard rubber rebate in the front of the slide, against which the glass is pressed, and behind the film is another washer which clips the plate all round the edges so as to form, with the recessed back, a shallow trough into which the mercury is allowed to flow from a reservoir bottle connected with the slide by a rubber tube.
Nothing but chemically pure mercury must be used, but even this is liable to oxidation, and this causes streaks and marks on the film on development. It is advisable, therefore, either occasionally to treat the mercury with sulphuric acid, which dissolves the oxide, or, better still, to form a bag of fine chamois leather by gathering together the ends and then shake the mercury to and fro in this, the leather having been thoroughly freed from grease by washing with soap and water and petroleum ether. In all cases it is advisable to rub gently the gelatine film of the plate after removing it from the dark slide with a tuft of cotton-wool, to remove any adherent globule of mercury. In filling the slide with mercury great care should be taken to obtain an even flow of the latter, as stoppage in the flow will generally show as a line across the plate.
A finished heliochrome, when examined by looking through it, generally appears as a more or less dense negative of a brownish or brownish black hue, and it is only when looking at the film at a particular angle that the colours can be seen. As a rule they appear very pale, and they are actually diluted with white light reflected from the front surface of the film. For pre liminary examination of the results the hello chrome may be immersed at an angle in a vessel containing benzole, but for permanent examin ation it is advisable to cement to the film (with Canada balsam or gum styra) a prism of glass of about io° angle.
The pictures must be viewed by parallel light, and all side light cut off ; this may be effected by cutting a hole in a window shutter and stand ing with the back to the hole with the hello chrome held in the hand at arm's length and shifted till it reflects the sky light. Obviously, somewhat similar results can be obtained by enclosing the heliochrome in a box and reflecting sky light on to it by a mirror.
For projection on to a screen by means of a lantern the aphengescope may be used.