Iodine by Multiplex Excitation. 677 



was obtained by photographing the spectra with a concave 

 grating of 2 metres radius of curvature, very bright in the 

 first order, ruled with 15,000 lines to the inch. The absorp- 

 tion spectrum was studied visually in the fourth order 

 spectrum of a six-inch plane grating with the large spectro- 

 meter previously mentioned. The resolving power was 

 300,000, and the grating easily separated lines only 0*03 

 Angstrom units apart. Photographs were made with this 

 grating, but they did not show as much fine detail as could 

 be observed visually and measured with the eyepiece micro- 

 meter, on account of small tremors which are never wholly 

 absent in Baltimore, even late at ni^ht. 



The fine groups of absorption-lines of iodine which fall 

 within the range covered by the green and two yellow 

 emission-lines of the mercury arc were first observed and 

 studied with a large echelon grating, but subsequent work 

 with the large plane grating showed that the results yielded 

 by the echelon had been wrongly interpreted, owing to the 

 overlapping of different orders, and they will not be further 

 discussed, as it was at once apparent that a six-inch plane 

 grating in the fourth order spectrum gave equal resolving 

 power, sharper definition, and results about the interpretation 

 of which there could be no doubt. In each case the spectrum 

 of a neon tube was photographed on the same plate super- 

 posed on the resonance spectrum, the length of the slit being 

 reduced, however, to make their identification certain. In 

 some cases the iron arc was impressed upon the plate as well, 

 as the neon tube gives but few lines in the green region, 

 while very rich in lines in the yellow, orange, and red. In 

 the case of the grating photographs the comparison spectrum 

 was not superposed on the resonance spectrum, the usual 

 method of a rotating slot being employed. 



The ^.0-foot Spectrograph. 



The discovery of the satellite lines which accompany the 

 resonance lines, and the change in their position and intensity 

 which resulted from changes in the distribution of the 

 intensity within the exciting line, made a careful photographic 

 study of the absorption spectrum of the iodine and of the 

 emission spectrum of the mercury arc much to be desired, 

 I accordingly fitted up during the past summer a plane 

 grating spectrograph of 12' 5 metres focal length. As this 

 spectrograph appears to be, with the possible exception of 

 Professor Michelson's ten-inch grating instrument, the largest 

 and most powerful in the world, a description of the method 

 of mounting may be of interest. The grating is a plane one 



Phil. Mag. S. 6. Vol. 24. No. 142. Oct. 1912. 2 Y 



