ASTRONOMY — HALK. 69 



division), have related especially to a comparison of hydrogen and calcium 

 flocculi, and an examination of stereoscopic solar images. The Zeiss 

 stereocomparator has proved of the greatest value in this work. 



Stereoscopic comparisons of two calcium photographs, made at intervals 

 of from one to ten hours, show the spherical surface of the sun in 

 relief, and are likely to prove of considerable service in our future work. 

 With the monocular micrometer of the stereocomparator, a comparison 

 of hydrogen and calcium photographs of the same date has shown a 

 relative displacement of the hydrogen flocculi towards the sun's limb, 

 presumably due to their greater height above the photosphere. This 

 result confirms my previous views, based upon other reasoning, as to 

 the level of the hydrogen flocculi. A further confirmation of these views 

 has been afforded by a study of the spectrum of the dark calcium floc- 

 culi, which are occasionally found at points on the solar surface occupied 

 by unusually dark hydrogen flocculi. It had previously been assumed 

 that these dark hydrogen flocculi were at a height above the photo- 

 sphere corresponding to the dark H3 and Kg lines ; in other words, in the 

 upper and cooler region of the chromosphere. It is now found that the 

 H3 and Kg lines are much strengthened and widened in these peculiar 

 dark flocculi, and are also accompanied by a marked strengthening of 

 the hydrogen lines. They are thus high-level phenomena and generally 

 occur, in all probability, above the region of the bright Hg and K2 flocculi. 

 Indeed, they sometimes appear, in the photographs, to overlie these 

 bright flocculi. It is a noteworthy fact that most of these singular dark 

 calcium flocculi are long and narrow in form ; the meaning of this is not 

 yet apparent. 



I have devoted much time and attention to a consideration of the best 

 methods of measuring and interpreting spectroheliograph plates, with 

 results which will be published soon. The globe-measuring machine 

 seems to satisfy all requirements for the determination of heliographic 

 positions; for the comparison (when used as a stereocomparator) of 

 photographs of the same and different dates ; and for other related pur- 

 poses. I am now engaged in perfecting a method of securing a measure 

 of the solar activity in regions 10 degrees square upon the solar surface, 

 as defined by the area and brightness of the calcium flocculi. It is 

 obvious that some method of determining the solar activity of a given 

 date, which shall be more satisfactory than that afforded by the total 

 area of sun-spots, is greatly needed. The spots are so small that, even 

 at times of maxima, the determination of their total area may involve a 

 considerable percentage of error. Moreover, at sun-spot minima, they 

 are sometimes absent from the disk for months together. The flocculi, 

 however, are always present, in greater or lesser degree, upon the solar 



