then allowed to dry. The label varnish is a commercial article,
or may be prepared with
Mastic ... ... ... ... ... 30 parts.
Oil of Lavender ... ... ... ... 5 ,,
Alcohol ............... 150 (l
Benzine ... ... ... ... ... 40 ..
this is allowed to stand for eight days with occasional agitation, and then decanted. This should only be applied to collotypes on chromo or baryta paper, and should be applied with a broad brush, and the prints dried in an oven or drying box. Grained copper, zinc and lead plates have also been used instead of glass for the support, but have not found much application com-mercially. Flexible supports have also been used, such as the so-called vegetable parchment.
Colour, Effect of, in Photography. See Isochromatic Photography.
Colour of the Film. This exercises a great effect upon the subsequent operation of printing. The yellow-stained film is the most non-actinic and slowest, the olive-greenish black being the quickest and giving the most brilliant prints.
Colour-Sensitometer. Abney's colour sensitometer in its simplest form consists of a series, or row, of coloured glasses, and a stepped sector-disc which rotates rapidly in front of these glasses; so controlling the light that the luminosity of each coloured pane, considered as a window, is equal. If a truly orthochromatic plate is exposed behind this system of coloured windows, it is obvious that all the windows will make equal impressions on the plate ; but no plate is truly orthochromatic, so coloured screens have to be adjusted to the plate. Assume, for example, that the blue window produces a greater effect on the plate than any other. A yellowish screen is interposed so as to reduce the effect of the blue. By repeated trials on this system a screen can be adjusted to the plate, so that the two together will be practically orthochromatic. In a similar way plates and screens can be adjusted to register any groupings of colour, or to produce any required pseudo-chromatic effect. In the same kind of way we can so adjust screens to plates as to register the three groupings of colour necessary in making the triplet of negatives required for the Du Hauron process of heliochromy. Abney's