BAND OF AMMONIA, AND ITS OCCURRENCE IN THE SOLAR SPECTRUM. 353 
flame produced in this way could be kept fairly steady, and had a small, but 
intensely bright, central core of a yellow colour, which was surrounded by a paler 
envelope. The flame was pointed in the direction of the collimator of the spectro¬ 
graph, and an image of the bright central core was focussed on the slit by a quartz lens. 
Photographs were first taken with a small quartz spectrograph giving a dispersion of 
about 60A per millimetre at X 3360, and with a quartz Littrow instrument giving a 
dispersion in the same region of 7 A per millimetre. 
In view of the need for the highest attainable resolution of the central parts of the 
band, an attempt was made to photograph the spectrum of the flame in the 3rd order 
of a 10-foot concave grating, in which the dispersion is 1‘85A per millimetre. An 
exposure of 12 hours, however, yielded only a feeble trace of the band, and it was 
evident that very much longer exposures would be necessary to give satisfactory 
photographs for measurement. Difficulties were anticipated in maintaining steady 
instrumental conditions, as the regular mounting is still detained in Russia, and only 
a temporary arrangement of the grating was available. 
Advantage was therefore taken of the possibility of producing the band in the 
electric arc, which had previously been noted in the course of experiments made for 
other purposes. In the first arrangement tried the arc was enclosed in a glass globe 
provided with side tubes for the admission of the electrodes, and with a quartz 
window through which the arc could be observed, as in a previous investigation on 
the spectrum of magnesium. # It was found, however, that the quartz window 
became obscured by the condensation of water-vapour, and it was therefore replaced 
* ‘ Phil. Trans.,’ A, vol. 209, p. 449 (1909). 
3 A 2 
