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

DRY PLATES, FILMS AND PAPERS 163 The upper curve represents the relative degrees of sensitivity of the human eye to light of various colours, the lower curve that of the ordinary photographic plate. A glance at this diagram shows that the ordinary plate is not sensitive to rays of less refrangibility than the yellow, that the blue to violet rays have the maximum action upon the plate, and that the action of the violet end of the visible spectrum by far exceeds that of the remaining portion. Hence it is that if one endeavours to photograph black letters upon a red surface, using for this purpose an ordinary dry plate, no letters will be visible upon the developed plate, the light reflected from the red and the black having apparently the same amount of action upon the sensitive film. If the coloured objects are to be represented properly, i.e. if each colour is to have its proper light value, some means must be taken to make the effect of the various coloured lights upon the plate correspond as nearly as possible in their relative intensity to the effect which those lights have upon the eye. As this is the case, some method has to be devised for enabling the plate to record the action of the rays from the red end of the spectrum, and at the same time to tone down the effect produced by rays from the violet end. The first of these changes can be brought about by the action of certain dyes upon the film. The dye can be added to the emulsion when the plates are being made and such plates will keep for a long time, or if greater colour sensitiveness is required, the colouring matter may be prepared in very dilute solution with ammonia, and the plates immersed in this solution. The plates, however, do not keep well when so treated. Such special colour-sensitive plates can be bought as orthochromatic, isochromatic, panchromatic plates, etc. A brief mention here of some of the red sensitizing