igi 



Kfiodiledge & SeleDtilie fieuis 



A MONTHLY JOURNAL OF SCIENCE. 



Conducted by MAJOR B. BADEN-POWELL, F.R.A.S., and E. S. GREW, M.A. 



Vol. II. No. 9. [NEW series.] 



AUGUST, 1905. 



SIXPENCE. 



CONTENTS.— Sec Page VII. 



The 



Sun. in Calcivim LigKt* 



By William J. S. Lockver, M.A., Ph.D., F.R.A.S. 



I. 

 Some time ago an account was given in these pages 

 (Vol. I., p 150), of some of the results which Prof. 

 Hale had secured with the spectroheliograph he had so 

 successfully designed and used in conjunction with the 

 great refractor of the Yerkes Observatory. 



This work, as I have pointed out elsewhere, marked 

 a new epoch in solar physics, for it suggested possible 

 fields for research which, up to that time, were not con- 

 sidered within the region of practical accomplishment. 

 Thus, for instance, it is now possible to determine the 

 distribution on the sun's disc and limb of such sub- 

 stances as calcium, hydrogen, iron, and many other 

 materials, the lines in the spectrum of which are 

 sufficiently strong in the solar spectrum. Not only can 

 this question of distribution be minutely studied, but 

 by securing photographs in different years the variation 

 of the areas covered by these substances from year to 

 year can be measured. In this wav we have a method 

 of estimating solar activity. Again, we are in the pre- 

 sence of a means of very considerably increasing our 

 knowledge of sunspot formation because spots give us 

 only a very brief span in the life history of a disturbed 

 region, which can now be photographically traced long 

 before any indication of a spot is detected and long 

 after the spot itself has disappeared. 



Further, a means is now afforded of rapidly securing 

 the forms and positions of prominences on the solar 

 disc at one exposure, either by using calcium, hydrogen, 

 or, possibly, other lines for the investigation. By 

 successive exposures on any particular portion of the 

 limb comparatively rapid changes in prominences can 

 also be photographically recorded. 



These and many others are some among the numer- 

 ous problems that are now waiting investigation by the 

 aid of this powerful instrument of research, so that 

 there is plenty of work for those students of Solar 

 Physics who wish to participate in this field of inquiry. 



At the present time there are not many of these 

 instruments at work, or even in existence. In addition 

 to those used by Prof. Hale in America, and M. 

 Deslandres at Meudon, in France, Mr. Evershed, in 

 England, has been securing some small scale pictures 

 during the last few years; while at Potsdam another 

 small instrument is mounted on an equatorial telescope. 

 At the Solar Physics Observatory, South Kensington, 



a somewhat larger instrument than the last two men- 

 tioned has been at work during the past year, and 

 nearly a duplicate of this has been despatched to India 

 and is now in working order at the Kodiakanal Solar 

 Physics Observatory. 



There is e\ery reason, then, to hope that before long 

 more instruments will soon be erected and set in 

 operation in order to assist in the accumulation of 

 material for increasing our knowledge of the physics 

 of the sun. 



Fig. I. The iz^-inch Taylor Photo=visual Lens and Support for form- 

 ing the Solar Ima^e on the Primary Slit of the Spectrohelio- 

 graph. 



In the following paragraphs it is proposed to briefly 

 describe the South Kensington instrument and to refer 

 at no great length to some of the results that have been 

 gleaned from the photographs that were secured during 

 the summer months of last year. A more complete 

 account will be found in the Monthly Notices of the 

 Royal Astronomical Society (Vol. Ixv., p. 473), in a 

 paper communicated by me during last March. 



Unlike the spectroheliographs employed at the Yerkes 

 and Potsdam Observatories, where both are worked in 

 conjunction with equatorial telescopes, the one at South 

 Kensington is so arranged that the solar image is 

 formed by a lens (Fig. i), on which sunlight is thrown 



