Or the following matt varnish : -
Sandarac ......... ...... 18 grs.
Mastic ............... 4 ,,
Ether... ... ... ... ... ... 200 mins.
Benzole ... ...... ... 80 to 100 ..
The more benzole the finer the matt surface obtained. Or an ordinary negative varnish, containing one per cent, of tartaric acid, and as much water as it will bear without becoming turbid, may be used. Another preparation is the following: -
Gelatine or glue ... ... ... ... 60 grs.
In water ............... 4 drms.
Boiled milk......... ... ... 2 drms.
melting by the aid of a gentle heat, and flowing over the glass.
Aids to Focussing. Oiling the screen renders the grain of the glass but very little perceptible. Or a better expedient still for use with the compound focusser is a microscopic covering glass cemented to the ground surface of the screen with a drop of Canada balsam ; a small cross should be marked in lead pencil on the ground surface before doing this or else, through the ready accommodation of the focus of the eye, this plan will prove a hindrance rather than a help. Both the cross and image should be in focus together.
Fog is one of the commonest of all faults with gelatine nega-tives, and is seen as a veil over the whole negative, or is a deposit of silver upon the shadows more or less dense according to the greatness or slightness of the fog. There are two great divisions of fog - chemical and light fog. The former is caused by errors in the manipulation of the plate manufacture, or in development; the second by the impact of light, as through some crevice in the camera or dark slide, or over-exposure.
Chemical Fog, from errors in manufacture, may be traced to several causes, the chief being an abnormal excess of silver nitrate in the emulsion, and also decomposed gelatine, caused by too long stewing, when the emulsion is made by the boiling process. If the fog is very bad, there is no cure for it, but when slight the emulsion may be squeezed through coarse canvas or