. The London, Edinburgh and Dublin philosophical magazine and journal of science. trum. The grating upon the whole isneither a very satisfactory nor an interesting one. It concen-trates light both to the right and left of the central image,giving the brilliant spectra of the first class alluded to in theprevious paper, at angles of 40° and 55° for normal each case the important thing to determine is the positionof the heat-ray spectra wdth respect to the blaze of light Energy Distribution of Diffraction Gratings. 893 (1st class spectra) reflected by the oblique edges of thegrooves
. The London, Edinburgh and Dublin philosophical magazine and journal of science. trum. The grating upon the whole isneither a very satisfactory nor an interesting one. It concen-trates light both to the right and left of the central image,giving the brilliant spectra of the first class alluded to in theprevious paper, at angles of 40° and 55° for normal each case the important thing to determine is the positionof the heat-ray spectra wdth respect to the blaze of light Energy Distribution of Diffraction Gratings. 893 (1st class spectra) reflected by the oblique edges of thegrooves. If one edge of the groove makes an angle of 20°with the original surface, and the light is incident normally,the blaze of light, or the oblique image as we will term ithereafter, is seen at an angle of 40°. In the case of thespectrometer readings, it will be found 20° from the centralimage, since, when the grating turns with the circle througha given angle, the ray turns through the doable angle. Theangular position of the oblique image on our chart (fig. 3) is Fier. 3J* 30° IS aoe IS9 0° 5° Cft^^te S therefore given by the angle of the edge of the groove. Ithas been indicated by brackets. In the case of very coarserulings it is merely an image of the source, but slightlydiffused by diffraction ; with the finer rulings we have thebroad maxima and minima of the first class (the positions ofthe superposed patterns due to the individual reflecting stripsor the so-called oblique images, are indicated by bracketsin fig. 3). -894 Profs. Trowbridge and Wood on Groove-Form and In the case of grating No. 4, the heat-ray spectra werefound between the oblique image and the true central image,as will be seen from the chart. The explanation of this is notclear and it seems probable that some grave mistake wasmade in its examination. It will be necessary to repeat the observations with thisgrating, for no spectra were found to the right of the centralimage, with the exception o
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