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298 THE ADVANCE OF PHOTOGRAPHY
the sun's disc, they are distinguished from the calcium flocculi by several important peculiarities.
In the first place most of them are dark where the corresponding calcium flocculi are bright ; then again they seem to obey a different law of rotation, for although the sunspots, faculae, and calcium flocculi all seem to show the polar retardation, this is not the case with the hydrogen flocculi. Lastly, and this is the most interesting fact of all, they exhibit a very decided definiteness of structure, which, as Professor Hale's photographs clearly show, is indicated by radial or curving lines, or by some such distribution of the minor flocculi as is exhibited by iron filings when placed in a magnetic field. This is shown in the accompanying plate, which is a reproduction of a photograph taken at the Mount Wilson Observatory.
This was first recognized at Mount Wilson in 1903. For the purpose of these observations, the hydrogen fines Ha, H/3, Hy, US are used. It is with the Ha lines since April 30, 1908, that the real advance in obtaining photographs showing definite structure has been made.
B}T observation of the spot photographs it is found that the spots in the northern hemisphere of the sun seem to indicate a cyclonic disturbance or vortex in which the motion is often, although not invariably, in a counterclockwise direction (see fig. 126).
In conjunction with these observations, a more extended study of sunspot spectra, in comparison with the ordinal solar spectra, has brought many interesting facts to light.
The great object in arranging apparatus for furthering the spectroscopic study of the sun has been to devise some means of employing a long-focus spectroscope, in conjunction with a long-focus telescope.
The method of overcoming this difficulty at the Mount Wilson Observatory is as follows : —
Two adjustable plane mirrors, each one foot in thickness, are so arranged that they can reflect a beam of sunlight