reproduce them by adding lights ; is it possible to do so by printing
with inks ? It is, at least, theoretically. The result, however, would
be a very dark picture of no use in practice, as it would require to be
looked at either in full sunlight or some equally powerful light.
Suppose, then, we take a card and first print it all over with an ink that absorbs all the spectrum except a narrow band in the red, another narrow band in the green, and a third in the violet (whether such an ink could be found is immaterial as our aim is purely theoretical). If the width and position of these bands are correct, i.e., if the colours left match the three sensation colours in hue, and their luminosities are properly proportioned, the card will now appear a neutral grey. In a powerful light it will appear white.- This is to be our white.
The inks must each absorb one of these bands of colour and leave the other two bands unaffected. The yellow ink will absorb the violet, the pink ink the green, and the blue ink the red band.
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Now if we print with these inks in amounts which are the comple- mentaries of the amounts indicated by Maxwell's curves, we shall be transmitting lights in amounts which are the same as those given by Maxwell's curves, and thus we shall exactly match all the colours.! A picture so obtained will be correct both in hue and luminosity,* but it will have to be examined in a very powerful light.
The three bands of colour left, referred to above, ought all to be very narrow so that the light may be approximately monochromatic.
But now suppose these bands not to be infinitely narrow. The light will no longer be monochromatic and a single band will excite more than one colour-sensation in our eyes. For instance, a band in the red unless in the very extreme red excites both the red and green sensations ; a band in the green excites all three (the red and violet will be nearly in the proportion to form white, and, if desired, the colour may be made equivalent to green and white by a proper selection of the width and position of the band) ; a band in the violet unless in the extreme violet will excite also red and green
t The curves referred to above are those in which equal heights of ordinate form white, e.g., Abney, loc. cit., p. 284, fig. 8.
- See Note 1, p. 27.
