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296 THE ADVANCE OF PHOTOGRAPHY
The primary slit, and the whole of the spectroscopic arrangements, move in a horizontal plane at right angles to the rays of light coming from the telescope, so each portion of the sun's image passes across the primary slit and an image of the sun as formed by some particular line of the spectrum will be produced on the sensitive plate.
It could be arranged that the image of the sun should move at a definite rate across the primary slit, the plate being made to move at a corresponding rate across the secondary slit.
In this case of course the spectroscopic part would be stationary ; this would be useful if the latter parts are so massive that their motion would cause the telescope to vibrate.
The lines which are most frequently used for this purpose are the " K " line of calcium and the " H " lines of hydrogen.
Photographs obtained by the " K " line show sunspots surrounded and frequently covered with vast clouds of calcium vapour, which there attain an elevation of several thousands of miles, but which must not be confused with the solar prominences, for the latter reach a much greater altitude. These cloud-like masses of calcium vapour are called calcium flocculi.
The next three illustrations will show the great change which is seen in the photographs when obtained by means of the spectroheliograph as compared with direct photographs.
The first (fig. 123) shows a reproduction of a direct photograph of an insignificant group of sunspots obtained under the excellent atmospheric conditions of the dry season at Mount Wilson Observatory.
The next (fig. 124) shows an enormous calcium flocculi occupying the same region of the sun, its form is, however, by no means remarkable, and certainly gives no indication of the phenomena brought to light by the Ha photograph as shown in fig. 125.