SPECTRO-PHOTOGRAPTIY.
31E.kSrP,EMENT OF CoLour, INTENSITIES.
By an ingenious invention of Sir W. Abney, any of the intensities of the various parts of the spectrum can be measured. The proportionate brightness of the dif ferent colours may be represented by a curve, as shown in Fig. 539, which pro As spectro-photography is dealt with in another section, only a brief description need he given here. The apparatus is shown in Fig. 591, and consists of an ad justable slit A at one end of a tube B, which carries a lens c capable of focus sing rays a mile or so away, thereby rendering them practically parallel. This tube, which is of the same length as the focus of the lens, is called a collimator. On a revolving table D is a glass prism or series of prisms E E ; for this work a quartz prism is preferable, as it transmits vides a most striking comparison of the relative visual intensities. The highest point reached by the curve represents the brightest or most luminous part of the spectrum. The blending of colour is so delicate that in ordinary circumstances it would be exceedingly difficult to locate any particular part of the spec trum ; but fortunately, at certain points, lines occur which are due to the presence of such metals as sodium, lithium, stron tium, etc. These serve as guides, as they always appear in the same place provided the same prism is used, thereby- affording a ready means of identifying any particu lar part of any colour. Each line has been given a letter ; the n line, for ex ample, always comes in the yellow, and occupies exactly the same place. The
line comes in the violet. The position of two such lines being found, the remainder of the spectrum may be readily plotted. Fig. 590 gives a diagrammatic representa tion of the spectrum. For such accurate observation it is necessary to use a spec troscope, an instrument extremely useful to the scientific photographer.
more of the ultra-violet rays, but a glass prism answers very well. F is a telescope capable of focussing the spectrum, and allowing certain parts of it to be examined critically. If a photograph is taken of the spectrum thus formed, and the propor tionate densities of its different parts accurately measured, it will be interesting to compare the curve which may be worked out from such a result with the visual curve above described. This curve will closely resemble that shown in Fig. 592. By comparing these curves a better impression may be obtained of the falsity of rendering to which the plate is liable nutler ordinary conditions. It will at once be seen that while the red and yellow have practically no effect, the blue and violet have proportionately too much action, whilst beyond the extreme limits of the violet will be found a similar action going on, apparently without light, but really due to the invisible ultra-violet or chemical rays.