710 PROGRESS OF ASTRONOMY FOR l«i»l AND 1X92. 



nciicc, ;iii(l I lie |n<»iiii!i('iicc iiimj^c is l>uilt tip iVoii! a succession of bright 

 line images of tlic slit. In tlie second iiielliod ])r(»])os('(l, the clock of 

 the equatorial is so adjusted that the linage of the sun is kept in a iixed 

 position. Tlie plate on the end of the collimatof which carries the slit, 

 is then slowly )no\'e<l across the sun's limb at the point where the 

 proniineuce is ])resent, and a second slit moving at the same speed 

 before a stationary i)lat<' excludes the light from the specti'um on either 

 side of the line in use, and reduces fogging to a minimum. 



In April, ISOl, Mr. Hale secured the first photograph of the sj^ectrnm 

 of a prominence obtained without an eclipse. This showed two very 

 strong, bright lines nearly at the centers of the dark solar bands Hand 

 K. The same lines were photographed on subseqent occasions, but it 

 was not until June 2.'>, that any new lines were discovered. On this 

 occasitm four lines were obtained in the ultra-violet — a number since 

 increased to six. Of these six, 5 lines belong, unmistakably, to the se- 

 ries of hydrogen lines discovered by l)i'. ITuggins, in the ultra-violet of 

 the sjiectra of Sirian stars. The sixth line forms a close double with 

 one of these hydrogen lines {a) but its origin has not yet been accounted 

 foi\ 



Mr. Hale's conclusion that 11 and K are not due to hydrogen, is abund- 

 antly confirmed by Prof. A'oung and also by M. Deslandres, sinc^ethe 

 measures have shown beyond a doubt that the "companion line to H," 

 and not H itself, is the one really due to hydrog'en. Mr. Hale and ]\r. 

 Deslandres ascribe these two giant bands of the solar spectrum to 

 calcium. 



Mr. Hale has also met with considerable success in photographing 

 the forms of solar prominences, some of the photographs showing a 

 satisfactory amount of detail. In one instance a prominence photo- 

 graphed at ivenwood was being sketched by Herr Feuyi at Kalocsa 

 at the same nu>ment of time, and drawing and ])hotograph are in close 

 accord. ^V suggestion by M. Deslandres that it inight be possible to 

 photograph the entire chromosphere at a single exposure has been 

 carried into effect by Mr. Hale, by means of a ''spectroheliograph," 

 in which the slit of the spectroscope is made to travel across the image 

 of the sun, and a precisely similar motion, but in an oi)posite direc- 

 tion, is given to a second slit neaily in the focus of the view telescope, 

 and so arranged that the K line of the spectrum of the fourth order 

 falls upon it. Since the K line is always bright in tlie spectrum of the 

 chromosphere and ])ronunences, it is easy, by shutting off the image of 

 the sun by means of a diaphragm, to build up a complete jncture of 

 the entire chromosphere an<l prominences, and so to produce what 

 may be described as an "artificial total solar ecli])se."' The discovery 

 Avliich Mr. Hale lias made that the H and K lines are always reversed 

 in the facuhe has enabled him to extend the application of this prin- 

 ciple. If the diaphragm covering the image of the sun be discarded 

 a photograph will be obtained, not merely of the chromosphere and 

 prominences, but of the disc of the sun itseTf, showing the spots and 



