A Manual Of Photography

THE AGENT PRODUCING CHEMICAL CHANGE. crimson ray, may be detected, by examining the whole through a deep blue glass ; and, by throwing the spectrum upon a piece of yellow paper, another ray appears at the violet extremity, named by Sir John Herschel the lavender ray. Yet more recently, Mr. Stokes has proved a most remarkable extension of the luminous rays. By throwing a prismatic spectrum upon a solution of quinine in diluted sulphuric acid, or an infusion of the bark of the horse-chesnut tree, an extra spectral ray, situated far beyond the violet, makes its appearance, proving an extension of light over a space which has hitherto been thought incapable of producing any luminous phenomena. The original spectrum of seven bands of colour was examined by Sir Isaac Newton, and that eminent philosopher determined that a given degree of refrangibility indicated a given colour; that the colour of a ray at once indicated its angle of refraction. Since the days of Newton, until our own time, this position had never been called in question; the seven rays were regarded as the primary colours of white light, and the law of Newton received as truth upon liis authority. Sir David Brewster lias, however, shown that this law will not stand the test of exami- nation. ’He has proved that the prismatic spectrum consists of three chromatic spectra overlapping each other, and that those three colours—red, yellow, and blue can be detected in cie1y part of the image. Sir John Herschel has added two rays to the luminous or visible spectrum,—thus making the number nine instead of seven ; but these can, equally with the others, be shown to be but combinations of the three primaries. There is, however, much reason to doubt if the new extra spectral blue rays can be comprehended within these three. The colours of Light will be rendered most, familiar by calling to memory the conditions of that very beautiful natural pheno- menon, the rainbow. The primary bow is usually accompanied by a secondary image, in which the order of the colours is re- versed. From close examination of the prismatic spectrum, I am disposed to believe that whenever we obtain this chromatic division of white light it is accompanied by a secondary spectrum, and that the real conditions of the colours are as follows:— The yellow is the most luminous ray, and the illuminating power diminishes on either side of it; on one side it blends with the blue, to form the green, and on the other with the red, giving rise to the orange ray. The blue diminishing in intensity sinks towards blackness, and thus produces the indigo, the extreme edge of which represents the limit of the ordinary spectrum at that end; as the outer edge of the red forms its limits, as far as the human eye is concerned, on the other. Beyond the indigo