18 



KNOWLEDGE 



[January 1, 1894. 



observed the faculsE reversal when at the limb without 

 finding any j)rominence over it, and I think I may say 

 that, as often as not, the calcium reversal ends abruptly 

 in the chromosphere. 



Again, the faculse are concentrated in the zones of spot 

 formation, and cover such vast areas as to present the 

 appearance in Prof. Hale's beautiful photograph of two 

 nearly continuous belts, one on each side of the equator, 

 forcibly reminding one, as Prof. Hale has himself remarked, 

 of the equatorial belts on Jupiter. The prominences, how- 

 ever, are by no means confined to this region ; in recent 

 years I have found them mostly developed in much higher 

 latitudes, in zones which appear to drift from mid-latitudes 

 towards the poles (see Asti-utiumi/ and Astyo-Physirs, No. 105, 

 p. 427). The prominences found in the faculous region, 

 say ± 10°, are very sparsely distributed in longitude, and 

 the area occupied by them is certainly much less than 

 that covered by faculiB. Thus I do not think that there 

 is any direct correspondence between these phenomena. 



The comparatively rare metallic and eruptive promi- 

 nences are, however, found almost exclusively in the spot 

 regions, and I think it highly probable that these are 

 developed from facuhv in disturbed regions, and it is to 

 these that I should attribute such phenomena as- the 

 sudden veiling of a spot by calcium vapour, so well shown 

 in the photographs taken by Prof. Hale on July 15th, 

 1892 yAstniwimi/ and AsDyi-FIii/sirs, No. 110), and I will 

 venture to predict that these eruptive reversals will show 

 also the companion line to H reversed ; thus distinguishing 

 them from the true facuhc. 



My conclusion, therefore, is that monochromatic photo- 

 graphs obtained by Prof. Hale's method in H or K light will 

 show faithfully all the true facuhe, but that occasionally 

 these forms will be masked by an overlying extension of 

 calcium vapour or by a prommence of exceptional brilliancy. 

 The ordinary prominences, I believe, cannot thus be 

 photographed in projection upon the disc. 



Yours truly, 



J. EVEESHED. 

 1 * I 



To the Editor of Knowledge. 



Gore Lodge, Glenageary, Co. Dublin, 

 5th December, 1893. 



Dear Sir, — Referring to your observations relative to my 

 moonlight photograph, which you reproduced in your issue 

 of this month, the camera used had no "cap" arrange- 

 ment, and the shutter was closed by me before removing 

 the camera from the position in which the exposure was 

 made. I feel perfectly convinced that the plate received 

 no such irregular exposure as you imagine ; but it has 

 been suggested to me that the lightning-like marks may 

 be due to some phosphorescent insect having crawled over 

 the lens or plate. Yours truly, 



Robert R. Levingston. 



[If a phosphorescent insect had crawled over the lens 

 during the exposure, its light might have thrown a faint 

 general illumination on the whole plate ; but no image of 

 the insect would have been thrown on the plate. The'^fact 

 that the lightning-like trace does not extend to the edge 

 of the plate, but stops at the line which marks the edge of 

 the dark side, seems to show that the line was traced 

 while the plate was exposed in the dark slide. Supposing 

 that the light from a luminous insect crawling over the 

 plate was sufficient to leave a photographic trace, one 

 would expect the photographic action to be brightest at 

 the centre of the path and to decrease in brightness 

 gradually towards its edges ; but the band on Mr. Leving- 

 ston'3 plate has sharp edges. On the other hand, if the 



light was derived from a firefly in front of the camera, it 

 must have been at a considerable distance, or its image 

 thrown on the plate would have been out of focus and soft 

 at the edges, in a camera which gave sharp images of a 

 distant mountain ; but it is hardly conceivable that a 

 distant firefly would have been sufiieiently bright to give 

 such a trace, and we know from the breadth of the trace 

 that the luminous object which caused it must have sub- 

 tended about half a degree in diameter as seen from the 

 camera. — A. C. Ranyard.] 



THE SOLAR FACUL^. 



By Prof. Geo. E. Hale. 



THE almost simultaneous discovery of the doubly- 

 reversed H and K lines in the spectrum of the 

 faculfB, made by M. Deslandres and myself in 

 1891, was not altogether unexpected. Several 

 years earlier, Prof. Young had seen this bright 

 pair of lines in the spectrum of spots, but their position at 

 the extreme limit of the visible spectrum made satisfactory 

 observation imposible. Prof. Young found, however, that 

 the bright lines were not confined to the spot itself, but 

 extended on to the disc for a considerable distance. 



With the application of photography the difiiculties of 

 visual observation disappeared, and the bright lines were 

 found, not only in the neighbourhood of spots, but also in 

 extensive regions irregularly distributed over the solar 

 disc. A dark central line of double reversal, which had 

 escaped the eye of the observer, was also clearly registered 

 upon the photographic plate. With a suflicient dispersion 

 it is occasionally found that the doubly-reversed lines 

 extend entirely across the sun. They are not uniformly 

 bright, but have alternate maxima and minima of 

 intensity. In the minima the lines sometimes seem to 

 disappear completely, but it appears probable that with 

 sufficient dispersion they could be photographed at any 

 point on the disc. The accompanying cut, which is a 

 direct reproduction of an enlarged photograph of the K 



Sim's 

 Limb. 



line (H is just on the edge of the field), will perhaps give 

 an idea of the character of the double reversal. On the 

 original photograph the lines are unbroken throughout 

 their whole length, and of nearly uniform intensity on 

 either side of a strongly-marked maximum. H and K are 



