COLOUR SENSITOMETRY Soon after the introduction of the colour sensitive plate some method was found needful for expressing the added colour sensitiveness, and this was effected by exposing a plate in the spectrograph and estimating the densities by visual examination or merely drawing a graphic curve, a test of great unreliability, still further complicated by the fact that prismatic spectro graphs were used. Later, plates were exposed to isolated patches of monochromatic light, a curve being obtained from the resulting negatives. This method was still further improved by using a spectrum and varying intensities of white light and obtaining an interpolation curve.
The great disadvantage •of the spectrographic method •is that it is not capable cif brief and common:1y understood =expression, and therefore many attempts have been made to obtain sensito metric tests by means of Charts of coloured pig ments, which are open to the most serious objection that the pigmentary colours reflect an enormous amount of white light, and whilst there is no object in nature that does not reflect white light, what the worker really desires to measure is the true increase in sensitiveness to a pure colour. Abney's colour sensitometer said the Chapman-Jones plate tester consist of coloured glasses or gelatines of equal luminosity, transmitting either small or broad isolated patches of the spectrum, and the densities obtain able can then be read and charted. Later, Eder and others divided the spectrum into three broad regions, the one including the blue and violet mp to about A 5,000, which is practically the sensitiveness of the ordinary emulsion ; a second region extending from the blue through the green to the yellow up to A 5,900 ; and a third, used only for panchromatic plates, extend ing through the red. Eder utilised the Scheirer sensitometer, and thus expressed mimetically the actual ratio of speeds of the non-colour sensitised emulsion and the added colour sensi tiveness.
This method has been still further extended by Mees and Sheppard to the Hurter and Driffield system, and is the most practical. The following are briefly the main features of it : the H. & D. sector wheel and the screened acetylene light (see " Plate Testing ") are used, and between the light source and the sector wheel are insetted absorbent solutions which limit the active light to particular regions of the spectrum. For test ing commercial iso- or orthochromatic plates, a yellow and a blue filter (Eder) are used ; the yellow filter consists of a 4 per cent. solution of potas
sium chromate (not bichromate) in a thickness of i cm. The blue filter is a a per cent, solution of amrnonio - sulphate of copper, also in s cm. thickness. The plate to be tested is exposed behind these two filters, and after develop ment the inertias are found in the usual way, and the result or ratio termed x (chr6ma, a colour), and is— yellow inertia blue sensitiveness blue inertia — yellow sensitiveness' For instance, a commercial iso plate was found to have an inertia of o•34 behind the blue screen and an inertia of 4.8 behind the yellow screen ; then— yellow inertia sensitiveness roo blue inertia o•34 — yellow sensitiveness 7•1 —X For panchromatic plates, it is essential to know also the increased sensitiveness to red ; and Mees and Sheppard use three screens as follows : the blue screen is Eder's given above, which passes the violet and blue up to A 5,000 ; the green screen, which passes from A 5,90o to A 5,000, consists of Eder's chromate screen given above °plus a screen of i cm. thickness of a -saturated solution of copper acetate ; the red screen is 'made with rose Bengal and tar trazine : Rose Bengal . . 48 grs. 5 g.
Tartrazine . . 96 „ so Distilled water to . zo oz. s,000 ccs.
Gelatine . . 728 grs. 73 g.
Allow 20 minims, or 1.25 ccs., to every square inch of glass.
In all cases the screened acetylene light has been used, and whilst this does not give the absolute inertias of the plate, the ratios of colour sensitiveness are preserved. For three colour work the actual filters to be employed may be used in the same way, and if an exposure be made without a filter the necessary increase of exposure for the blue-violet filter over the unscreened plate can readily be found. Con siderable influence is exercised naturally upon the results obtained by the nature of the light employed, and one can easily understand that the standard light should be, if possible, daylight of constant spectral composition, or a secondary standard with as near as possible the same spec tral composition ; for if the light be yellow, with a decided paucity in violet and blue rays, and corresponding richness in yellow and orange, the colour sensitive plate would show a much higher colour sensitiveness, which would be totally misleading. E. J. W.