NO. 1865 ami KNOVVIvEDGE OF THIC SUN HALE 351 



closed, excluding from the plate all light except that which conies 

 through the slit. An electric motor is then started, causing the iron 

 bed-plate, which is mounted on steel balls and carries the two slits, 

 the lenses and the prism-train, to move at a uniform rate across the 

 solar image. 



Plate XXXII reproduces a photograph made in this way, for com- 

 parison with a direct photograph (Plate xxviii) showing the Sun as 

 it appears to the eye in the telescope. The luminous clouds of calcium 

 vapor, or "flocculi," are well shown on the monochromatic image, but 

 do not appear in the direct photograph. It will therefore be recog- 

 nized that this method opens up an extensive field, by permitting the 

 invisible phenomena of the solar atmosphere to be investigated. The 

 wide range of the new information thus to be derived will be ap- 

 preciated when it is remembered that by photographing the Sun 

 with the lines of hydrogen, iron, sodium, magnesium, or any other 

 element represented among the thousands of lines of the solar spec- 

 trum, the distribution of the corresponding vapor can be recorded. 

 For example, Plate xxxiii is a picture of the hydrogen flocculi, made 

 six minutes after the calcium image in Plate xxxii was obtained. It 

 will be seen that most of the hydrogen clouds, instead of giving bright 

 images like those obtained with calcium, are comparatively dark, 

 though certain eruptive phenomena and regions in the neighborhood 

 of Sun-spots appear bright on the hydrogen plates. This spectre- 

 heliograph is also used to photograph the iron vapors in the Sun, 

 but, as will be explained later, a larger instrument is required to yield 

 satisfactory solar photographs with the narrower lines of other ele- 

 ments. 



The 5-foot spectroheliograph has been in regular use with the 

 Snow telescope since October, 1905. Photographs of the Sun are 

 made with the calcium, hydrogen and iron lines every clear day, 

 both in the morning and in the afternoon. About 3,700 negatives 

 thus obtained give a connected history of the Sun during the period 

 in question, and provide the material for such investigations as will 

 now be described. 



The first use of these plates that suggests itself is a study of the 

 solar rotation as determined by the rate of motion of the flocculi. 

 The flocculi change more or less in form from hour to hour, but some 

 of them may be identified on plates taken on several consecutive 

 days. Two plates, taken about twenty-four hours apart, are closely 

 compared and only those flocculi which undergo small change of 

 form are marked for measurement. The process of measurement in- 

 volves the determination of the latitude and longitude of each of 



