$92 Profs. Trowbridge and Wood on Groove-Form and 



spectra were caused to pass across the bolometer strip by 

 revolving the grating, which made the incidence angle vary, 

 and necessitated the use of the formula for fixed telescope 

 and collimator and rotating grating. Each grating was 

 studied with the quartz residual rays, and with the C0 2 

 radiations, and the curves representing intensity distribution 

 plotted. Ihe areas of the curves were measured with a 

 planimeter to determine the total energy in each spectrum. 

 The "central image" curve was always very much higher 

 than any spectrum curve, but owing to its narrowness 

 frequently contained much less energy ; in other words, we 

 cannot take the deflexion at the central image, and in a 

 given spectrum, as a measure of the energy distribution, 

 since the radiation is not monochromatic. The curves were 

 plotted on large sheets of coordinate paper, and cannot be 

 reproduced very well, even on a greatly reduced scale. It 

 has seemed best, therefore, to make a small chart, showing 

 in a rather qualitative manner the positions and magnitudes 

 of the spectra of different orders, obtained with each grating. 

 Potted lines represent the C0 2 radiation (wave-length 4*3), 

 solid lines the quartz rays (wave-length S'G). To save space 

 the central image curve, the height of which is sometimes 

 To or 80 times the width at the base, is shown on a much 

 -mailer scale. 



We will now take up the gratings individually. 



Grating No. 4. Constant, 0*0265. 



This grating was ruled on Rowland's oldest machine fitted 

 with a 15-tooth cam. The constant is therefore 15 times as 

 large as that of the usual gratings. The reflecting planes 

 made angles of 20°'5 and 27°'5 with the original surface. 

 Examination with the microscope, by the red and green 

 light method, described in the paper on the echelette grating, 

 showed that both reflecting surfaces were good, and that the 

 grooves were separated by strips of the original surface, 

 which appeared black under the microscope, and were of 

 such a width that the 27 0, 5 edge (red) plus the black strip, 

 was equal to the width of the 20 o, 5 edge (green), a matter 

 of importance in connexion with the disappearance of the 

 second order spectrum. The grating upon the whole is 

 neither 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 the 

 previous paper, at angles of 40° and 55° for normal incidence. 

 In each case the important thing to determine is the position 

 of the heat-ray spectra wdth respect to the blaze of light 



