215 



KDooiledge & Seientlfle tms 



A MONTHLY JOURNAL OF SCIENCE. 



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



Vol. II. No. 10. [new series.] SEPTEMBER, 1905. [stalSners' Haii ] SIXPENCE NET. 



CONTENTS.See Page VII. 



The 



Sun. in Calcivim Light. 



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



II. 



The loaded plate-holder is next slid into its carrier, 

 and both secondary slit and carrier are securely 

 wrapped in velvet as is consistent with the necessary 

 freedom of relative movement during the exposure. 

 The window blinds in the room containing the instru- 

 ment are then closed to keep out daylight as much as 

 possible. By pushing the whole upper framework to- 

 wards the east, the solar image is made to lie a little 

 to the west of the primary slit. The length of exposure 

 required is now judged by the brightness of the solar 

 image, and the rate of movement adjusted according to 

 a reading taken from a table giving the lengths of 

 " runs " corresponding to the temperature of the oil. 

 The dark slide is next opened and a suitable moment 

 for exposure awaited. When this opportunity occurs 

 the shutter behind the primary slit is opened and the 

 framework released by a starting handle. 



The primary slit then moves over the fixed solar 

 image and simultaneously with it the secondary slit 

 passes over the fixed photographic plate; the " K " 

 image is thus built up in the fcrm of a disc. The time 

 of transit of the slit over the image is indexed as the 

 " run," and when completed the slit shutter and plate 

 are closed. 



The operations for obtaining the photographs of the 

 prominences round the limb are very similar to the 

 above. The solar image falling on the primary slit is 

 blocked out by means of a metal disc of the same size 

 as this image, and a much longer exposure is given. 

 The ratio of the length of a " disc " and " limb " ex- 

 posure is about as i to 60. Under very favourable 

 circumstances a " disc " exposure lasts about 15 

 seconds. 



By taking a limb photograph first, and then removing 

 the metal disc and making another " run " for the 

 " disc," a composite picture on one plate is secured. 



Since this spectroheliograph has only been working 

 efficiently since the spring of last year (the recent winter 

 months being excluded as the low altitude of the sun in 

 London during this period renders this kind of work 

 almost impossible), the data at present available for 

 discussion are not very considerable. 



It will, however, not be without interest to refer to 

 some of the photographs obtained, which will serve to 

 illustrate not only the quality of the negatives secured, 

 but the different branches of work which such a series 

 of photographs as previously mentioned open up. 



In the accompanying illustrations will be found two 

 enlarged reproductions, one of the solar disc in " K " 

 light, taken on September 20, 1904 (Plate II.), and 

 another of a composite picture showing the limb and 

 disc taken on August 29 of the same vear (Plate III.). 



From a general examination of a great number of 

 the " disc " negatives it has been noted that over the 

 whole solar surface there is always a very distinct 

 " mottling " extending even to the solar poles. 



Illustrating a striking change in a prominence after 

 an interval of one hour. July 14. IQ04. 



About the equatorial regions this mottling seems in 

 places to be of an enlarged nature and unevenly dis- 

 tributed in longitude. In regions of apparently greater 

 disturbance the bright portions of this mottling become 

 amalgamated and produce the calcium clouds or 

 " flocculi," as termed by Prof. Hale. The type of 

 formation of these flocculi can be gathered from the 

 illustration (Plate I., Fig. 2) given last month. A 

 bright nucleus with radiating bright branches is not an 

 uncommon feature in a great number of the plates 

 examined. 



It is in these larger flocculus regions that spots are 

 observed. There can be flocculi without spots and 

 flocculi with spots, but, so far as the photographs have 

 shown, never spots without flocculi. The duration of 

 a spot is, further, only a brief interval in the life history 

 of a flocculus, so that to study the formation of spots 

 their relation to flocculi must be taken into account. 

 That an intimate connection in addition to that men- 

 tioned above does exist, is indicated by the fact that 

 spots appear more generally to precede the apparent 

 trailing masses of flocculi with respect to the solar 



