THE WORK OF CLERK-MAXIVELL.
Professor Clerk-Maxwell, in 1861, in a lecture delivered at the Royal Institution, practically demonstrated the truth of the Young-Helmholtz theory by means of three optical lanterns throwing light of the three primary colours. It was shown to be possible to combine the three coloured lights, so as to produce white, or by mix clown the principle that our eyes are capable of receiving three distinct forms of stimulus, or primary colour sensations —the red, the green, and the blue-violet. Whether the retina possesses three different groups of nerves, or whether one set of nerves can receive three distinct impressions, is a matter of uncertainty. It has, however, been proved that by mixtures of two or all of the three colours corresponding to the three primary sensa tions, in correct proportions, the impres sion of any other required tint or colour may be successfully made. As a matter of fact, colour in itself has no real ex istence. It is simply because different objects absorb certain rays of light, and reflect others to our eyes, which are cap able of exciting the nerves of colour sen sation in a particular manner, that the objects give the impression of possessing colour. It is quite possible for an , in dividual to be deficient or lacking in the nerves of a certain colour sensation, in ing them in suitable proportions to obtain any required tint. Clerk-Maxwell also succeeded in obtaining three different photographs of a coloured object, taken through the three colour screens, thus representing the red, the green, and the blue parts respectively of the image.
These three photographs, when shown in the lantern and superposed on each other, gave an image reproducing as far as possible the tints of the original. The most valuable feature of the work of Clerk-Maxwell, however, is his proof that the primary colour sensations are excited by mixtures of colours, and not simply by pure red, green, and blue. By means of curves, he was able to show exactly those parts of the spectrum which took part in exciting the three fundamental sensations (see Fig. 568). In the diagram it will be noticed that the red sensation curve includes not only red, but orange, yellow, and yellow-green ; the green curve proceeds from the orange through the yellow, green, and green-blue ; while the blue sensation is aroused by the blue green, blue, and violet rays of the spectrum. In 1865, Henry Collen, in a contribution to " The British Journal of Photography," suggested that three nega tives might be taken representing the three primary colour sensations, and from those three coloured positives, which superposed would truthfully reproduce the tints of the original. It was not, how ever, till much later that any actual work was done in this direction.