. The book of photography; practical, theoretical and applied. are an odd number of half-periods apart, give no resultant illumina-tion ; (ii.) two equal sets of waves, aneven number of half-periods apart, give amaximum illumination ; (iii.) the two wave-lengths must be equal to produce thisresult. The Diffraction Grating. The case of parallel rays of light fallingon a series of equally-spaced long and SPECTROPHOTOGRAPHY. 575 narrow rectangular apertures may benow considered. This arrangement ofapertures is termed a grating. By thewave theory it can be proved that thelight which enters slit s
. The book of photography; practical, theoretical and applied. are an odd number of half-periods apart, give no resultant illumina-tion ; (ii.) two equal sets of waves, aneven number of half-periods apart, give amaximum illumination ; (iii.) the two wave-lengths must be equal to produce thisresult. The Diffraction Grating. The case of parallel rays of light fallingon a series of equally-spaced long and SPECTROPHOTOGRAPHY. 575 narrow rectangular apertures may benow considered. This arrangement ofapertures is termed a grating. By thewave theory it can be proved that thelight which enters slit s (Fig. 771) takesthe same time to get to the gratingalong any of the paths through the lens the light is entirely from one small odd number of half wave-lengths behindthat coming from b, there will be no re-sultant light at z from these two aper-tures, and therefore no image. As thesame thing happens with each pair ofapertures, no image at all will be formedin this direction. If the difference of pathbe an even nvmiber of half wave-lengths COLLIMATOR. Fig. 771.—Formation of Spectrum. source, it will all be in the same phase ofvibration at the time it reaches thegrating. This idea is usually expressedby saying that the wave front is parallelto the surface of the grating. Since thelight is of small wave-length, as it passesthrough the slits it diffracts practically inall directions from each slit. In the dia-gram, therefore, for the sake of sim-plicity, but one colour has been considered,and only one directionfor that colour. Oninterposing the lens right angles to theseparallel rays, a realimage of the slit willbe obtained at z undercertain conditions. Thelens is now parallel to the line K b^. Consider the light passingthrough a to Oj. The light at point b isultimately added to that at bj to makethe image at the principal focus of l,^.But these two beams are not obliged toarrive at the image in the same these two corresponding raysthere
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