1. Indigo. 2. Antwerp Blue. 3. Cobalt. 4. French Ultramarine. Fig. 22.
Fig. 23.—Method of obtaining a Colour Template.
From curves such as these we are able to produce the colour of the pigment on the screen from the spectrum itself. This is a useful proof of the truth of the measurements made. To do this we must mark off on a card (Fig. 23) the absolute scale of the spectrum along the radius of a circle, and draw circles at the various points of the scale from its centre. From the same centre we must draw lines at angles to the fixed radius corresponding to the various apertures of the sectors required at the various points of the scale to measure the light reflected from a pigment. Where each radial line cuts the circle drawn through the particular point of the scale to which its angle has reference, gives us points which joined give a curved figure. Such a figure, when cut out and rotated in front of the spectrum in the proper position (as for instance by making the D sodium line correspond with that on the scale), will cut off exactly the same proportion of each colour that the pigment absorbs. The spectrum, when recombined, should give a patch of the exact colour of that measured. The spectrum must be made narrow, as the template is only theoretically correct for a spectrum of the width of a line, as can be readily seen.
Templates like these will always enable any colour to be reproduced on the screen, and if the light be used for the spectrum in which the colour has to be viewed, be it sunlight, gaslight, starlight—whatever light it is—the colour obtained will be that which the pigment would reflect if it were viewed in that light.
The identity of the colour produced on the screen by this plan with that measured, can be readily seen by placing the latter in the reflected beam of white light alongside the coloured patch formed on the white surface.
Fig. 24.—Template of Carmine.
In Fig. 24 we have a mask or template of carmine, which was used for determining if the measurements were right. The black fingerlike-looking space on the right was the amount of red reflected light, and the other that of the blue and violet; scarcely any light at all was reflected from the green part of the spectrum.