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ASTRONOMICAL PHOTOGRAPHY 297
In the solar physics laboratory at Kensington the image of the sun as formed on the plate of the primary slit is just over 2 inches in diameter, and the primary slit has a length of 3 inches. The jaws of the secondary slit are 3J inches in length and are slightly curved, the curvature corresponding to that of the " K " line of calcium.
It is usual to arrange a needle or some similar object at the secondary slit so as to form a datum line on the photograph.
When a photograph of the sun's limb is required the amount of light available is so reduced that the time of exposure must be at least sixty times that required for a disc photograph.
Sometimes photographs are taken of the solar prominences by this means, then it becomes necessary to cover the sun's image with a metal disc, and to give the moving platform a greater degree of motion as well as a much slower rate. It is usual to subsequently expose the same plate, after removal of the disc, so as to obtain a composite picture showing disc and limb with prominences.
Spectro-heliograph Work at Mount Wilson. — Of all the workers in this sphere of astronomical work Professor Hale of the Mount Wilson Observatory, U.S.A., is perhaps the foremost. He has, by means of photographs obtained in this manner, made extremely careful measurements of the daily motion in longitude of the calcium flocculi, and by these measurements has redetermined the times of the sun's rotation.
He has likewise investigated their distribution in latitude and longitude, and the forms of these flocculi at different levels.
By this means he has come to the conclusion that these calcium flocculi do not indicate the existence of currents in the solar atmosphere.
He has similarly observed the hydrogen flocculi, and finds that, while these occupy the same general regions of