AN ECLIPSE SPECTROSCOPE

Abstract

SOME little time ago I communicated to the Royal Society a suggestion for the use of Mr. Ruther-furd's reflection gratings in obtaining photographs of the coming eclipse. The plan suggested was that the grating should be placed short of the focal point of a telescope, and at right angles to its axis, and that the diffracted images of the chromosphere should be received on photographic plates adjusted for the different orders of spectra on either side the axis. I am glad to learn from Prof. Newcomb that the value of this method of observation will probably be tested by Prof. Young, who is in charge of one of the six expeditions already organised to observe the eclipse. The chief defect in this mode of observation lies in the difficulty of determining the position of the lines photographed, supposing the chromospheric spectrum to vary considerably from the ordinary solar one so far as the intensity of the lines is concerned; and as it seemed desirable that these gratings should be utilised for less serious attacks, I have recently been endeavouring to see if the method can be improved.