i2 POPULAR SCIENCE MONTHLY. 



spectroheliograph. Such a photograph is reproduced in Fig. 4. Its 

 most conspicuous features are the numerous dark spots scattered over 

 the sun's surface; these are the well known sun-spots. Near the edge 

 of the sun there may be seen certain bright regions, which are known 

 as faculse. The calcium regions above referred to are usually associ- 

 ated with the faculas, but they lie above them, and they give no trace 

 of their existence on ordinary photographs, like the one in Fig. 1, or 

 to the eye when observing the sun through a telescope. 



The results of the first experiments, which were made at the begin- 

 ning of 1892, were such as to justify fully the expectations that had 

 been entertained. It was at once found possible to record the forms, 

 not only of the brilliant clouds of calcium vapor associated with the 

 faculse, and occurring in the vicinity of sun-spots, but also of a reticu- 

 lated structure extending over the entire surface of the sun. The 

 earliest applications of the method were made in the study of the great 

 sun-spot of February, 1892, which, through the great scale of the phe- 

 nomena it exhibited, and the rapid changes that resulted from its ex- 

 ceptional activity, afforded the very conditions required to bring out 

 the peculiar advantages of the spectroheliograph. In the systematic 

 use of the instrument continued at the Kenwood Observatory through 

 the following years, a great variety of solar phenomena were recorded, 

 and the changes which they underwent from day to day— sometimes, 

 in the more violent eruptions, from minute to minute — were registered 

 in permanent form for careful study. During this period, which ended 

 with the transfer of the Kenwood instruments to the Yerkes Observa- 

 tory, over 3,000 photographs of solar phenomena were secured. From 

 a systematic study of these negatives, in the course of which the helio- 

 graphic latitude and longitude of the calcium regions in many parts 

 of the sun's disk were measured from day to day, a new determination 

 of the rate of the solar rotation in various latitudes has been made. 

 This shows that the calcium regions, like the sun-spots, complete a 

 rotation in much shorter time at the solar equator than at points nearer 

 the poles. In other words, the sun does not rotate as a solid body 

 would do, but rather like a ball of vapor, subject to laws which are 

 not yet understood. 



In this first period of its career the spectroheliograph had therefore 

 permitted the accomplishment of two principal objects. It had provided 

 a simple and accurate means of photographing the solar prominences 

 in full sunlight, which gave results hardly inferior to those obtained 

 during the brief moments of a total eclipse. It had also given a means 

 of recording a new class of phenomena, known previously to exist only 

 through glimpses of the bright calcium lines in the vicinity of sun- 

 spots, but wholly invisible to observation cither visually or on photo- 

 graphs taken by ordinary methods. It was not difficult to see, how- 



