Page:Proceedings of the Royal Society of London Vol 69.djvu/48

40 Thus, if the pink ink is only partly transparent in the blue and has not an abrupt absorption there it will not much matter, provided it is perfectly transparent in the red and orange, and has an absorption commencing suddenly in the yellow. Such an ink is Fleming's "Theoretical Eed," No. 1303.

The yellow must be perfectly transparent in the red, orange, yellow, and green with an absorption commencing in the blue ; but it does not matter if it is not very sudden.

The blue ink must be perfectly transparent in the green, blue-green, and blue ; there must be an abrupt absorption in the yellow. The violet does not matter so much. So far I have found no ink to fulfil these conditions. All the ordinary inks are too opaque in the green and blue-green.

9. The Luminosity as well as the Hue of the Spectrum must be Matched

everywhere.

It must not be forgotten that most natural colours are composed of a large range of spectrum colour, and the resultant hue will depend on the proportion in which these colours are compounded. Now although the eye is a bad judge of the luminosity of the colour, it is a very good judge of the resultant hue of a compound colour (and thus indirectly it is of course able to compare the relative luminosity of its components as far as this luminosity is due to colour as distinct from white). It follows that the component colours must be rendered in their correct proportions. When a negative is exposed through its light-filter the light that will reach it from a natural object will be the sum of all the colours that the object reflects, each rendered according to the curve of the filter. Suppose for instance that all the light from the orange to the green is reflected by the object as in the first figure.

Then if the second figure represents the curve of the light-filter, the area of the shaded portion gives the amount of transmitted light