The advance of photography : its history and modern applications (1911)

240 THE ADVANCE OF PHOTOGRAPHY to be placed before the sensitive plate when making the exposure. The importance of this was not recognized until Joly in England and MacDonough in America applied it in practice. This they did within a short time of one another. In 1881 Cros described a process very nearly the same as the later ones based on the Young-Helmholtz theory of colour vision. Thomas Young (1807) and V. Helmholtz. (1852) assumed that there are three primary light colours to which three different kinds of nerves in the retina respond. These three primary colours are red, green, and blue. Every colour in the spectrum was assumed to excite all the kinds of fibres, but some of them very feebly and others strongly. Thus, while red light excited the fibres sensitive to red most, yet those sensitive to blue and green were also feebly excited. This theory was afterwards (1861) elaborated by Clerk Maxwell, who put forward the idea that all colours could be produced from the three primar}^ colours — red, green, and blue. F. E. Ives of Philadelphia made use of this theory, and obtained three negatives, using respectively red, green, and blue colour screens. These negatives he developed and fixed in the ordinary manner, and then obtained three positives as transparencies. In order that it might be possible to view all three positives at the same time and in register, he invented a special apparatus which he named the KromsJcop. The diagram (102) will illustrate the principle on which the action of this instrument depends. A represents the position of a piece of red glass on which the positive made from the negative obtained with the red screen is placed. Similarly B and C represent positions of blue violet, and green glass with the suitable positive. E shows the position of the eye, and the arrows the direction of incident white light. L, M, N are three plane