I50 



KNOWLEDGE & SCIENTIFIC NEWS. 



[July, 1904. 



The Solar Atmosphere 

 at Different Levels. 



By E. Walter Maunder, F.R.A.S. 



In " Knowledge" for October, 1903, we published a fine 

 [jhotO'g;raph o^f the sun in K-line, taken oni April 27, 

 1903, with the Ruml'ord .spectroheliograph attached to 

 tire great 40-inch refractor of the Yerkes Observatory, 

 Ijy Professor Georg-e K. Hale and Mr. Ferdinand Eller- 

 nian. Tliis spectroheliograpli, a photoigraph o-f which 

 was given in the same number, lias a train of twoi prisms 

 of 60", set at minimum deviation, for the K-line. I he 

 collimator and camera lenses arc of the portrait lens 

 type by N'oigtlander with apertures <y{ 6^ inches. They 

 are of equal aperture and focal length (33 inches), and 

 niay be focusscd singly or together by means oif a rod 

 connecting- the pinions which move each lens in 

 its tube. The tubes of collimator and camera are 

 parallel tO' each Oithcr, the light from the colli- 

 mator being- reflected Ironi a, plane mirror on to 

 llie fust surface of the prism train. If required, a 

 much higher dispersion can be obtained by substituting- 

 a grating-, ruled with twenty thousand lines tO' the inch, 

 fi r the above mirror, the first order spectrum being em- 

 ployed. The second slit of the instrument is, of course, 

 placed close tO' the focus of the camera lens, and the 

 gieat 40-inch telescope is made toi move slowly ini de- 

 clination, by means oi a slo'W motion electric motor, the 

 sun's ima.ge consequently mo-ving- at a. uniform rate 

 acro'ss the first slit, whilst the photographic plate is at 

 the same time driven at the same rate across the second 

 slit by m.eans of a shaft led down the tube of the tele- 

 scope from, tlie motor. The moition of tliie focal imag;e 

 of the sun, produced by the motor, is aboiit one minute 

 of arc in. foiur seconds, when, one set of gears is em- 

 ployed, and in twenty-four seconds when another. The 

 two' slits are each 8 inches in length, and are given 

 the proper curvature necessary tO' eliminate the distor- 

 tion of the solar image. But, as the focal leng1;h of the 

 great refractor is 64 feet, and the image of the sun, in 

 the principal focus is consequently a little over 7 inches 

 in diameter, the aperture of the spectrohcliograph is 

 not quite sufficient for a full in-iage oif the sun, and 

 occasioned the falling- oiff in brig-htness at the twoi oppo- 

 site limbs of the sun, noticed in the plate published in 

 ■■ Knowledge," opposite p. 229 in. the last volume. 



At that time Professor Hale wrote: — '' By setting the 

 .second slit on various parts o,f the K-band it is possi- 

 ble toi photograph sections of the calcium flocculi at 

 different elevations above the photo«phere. Tfijs is due 

 I'l the fact that the width of the K-band is determined 

 by the density of the vapour; hence, if the slit is set near 

 the outer edge of the laroa.d band, it cam receive light 

 only from the calcium vapour, which is den.se enough 

 to produce a band of this width. When the slit is set 

 near llie centre of the band it receives light from all the 

 vapour King belo,w the corresponding level. But as the 

 v;ipour expands as it rises, a given photograph always 

 shows the structure corresponding tO' the lowest density 

 (h.igliicst level) of the calcium \,-ipoiir competent toi pro- 

 duce a line of the necessary width. I shall publish very 

 soon a. series of photographs shmving how spots are 

 successively co'vered by o,verh.a.nging calcium clouds in 

 photographs taken at different levels." 



'i'h.is promise has been fulfilled in the recently pub- 

 lished Memoir on the Rumford .Spcctrolieliograph whacb 



forms \'ol. 111., Part 1., of the Publications of the 

 V'erkes Observatory, from which we are enabled to re- 

 produce four photographs out of the great number by 

 which the Memoir is illustrated. The four chosen, 

 Figs. I to 4, represent the great spot group of 1903, 

 October 9, as photographed with the slit placed in 

 three different positions on the H-line of calcium and 

 upon the centre of the F-line of hydrogen. 



In the last number of " Knowledge," we reproduced a 

 portion of one of M. Janssen's superb photographs of 

 the solar surface, showing in a very distinct manner the 

 curious structure which, it presents. The minute 

 granulatio,n whichi the disk thus either shows tO' the eye 

 under the most perfect conditions of seeing, or reveals 

 to the photograpliic plate when the precautions taken 



Fig. 5. H and k lines on (he L>isl\, in the Chromosphere, and in a 

 Prominence ui). 



by M. Janssen are employed, has its parallel also In the 

 structure shown. Ijy the calcium " flocculi " (to adopt 

 the word suggested b\' Protessor Hale, and generally 

 accepted), re\ealed b\- the spectrciheliographi. l''rom 

 the greater difficulties of the work the granulation 

 shown by the flocculi is not in general so' minute as that 

 shown on |a.n.ssen's photographs, and in many cases 

 their granules appear to have run together to form the 

 great fleecv clouds sO' conspicuous in the photograph 

 reproduced in October of last year. Professor Hale, on 

 the working hypothesis which he at present employs, 

 considered the minute floccular granules as columns of 

 calcium v.apour, rising- abo've the columins of condensed 

 \ap0'urs of which photospheric granules are the 

 summits. The question arose whether these larger 

 calcium clouds wore made up of similar columns of 



