2^0 



KNOWLEDGE 



[October, 1908. 



group G, shown on p. 131, and it will be seen at ouce that 

 the smaller preceding spots of the group are entirely 

 hidden b_v the ramifications of the Flocculi. 



Prof. Hale, in a communication describing his instru- 

 ment and its work, writes : — " Bj setting the second slit on 



The Kumt'ord Spectrolicliogi-aph of the Yerkes Observatory. 



various parts of the K-band, it is possible to photograph 

 sections of the calcium flocculi at different elevations 

 above the photosphere. This is due to the fact that the 

 width of the K-band is determined bj the density of the 

 vaiiour ; hence, if the slit is set near the outer edge of the 

 broad band, it can receive light only from the calcium 

 vapour, which is dense enough to produce a baud of this 

 width. \Vhen the slit is set near the centre of the baud 

 it receives light from all the vapour lying below the 

 corresponding level. But as the vapour expands as it 

 rises, a given photograph always shows the structure 

 corresponding to the lowest density (highest level) of the 

 calcium vapour competent to produce a line of the necessary 

 width. I shall jiublish very soon a series of photographs 

 showing how spots are successively covered by overhanging 

 calcium clouds in photographs taken at different levels. 



" If sufficiently high dispersion is used, photographs can 

 be taken with any of the dark Unes of the solar spectrum. 

 For example, I have succeeded in obtaining hydrogen and 

 iron pictures by using a grating in place of the plane 

 mirror in the optical train of the spectroheliograph. The 

 grating gives a spectrum which is afterwards further 

 dispersed by the two prisms. The prisms also serve to 

 reduce greatly the diffuse light from the grating by 

 .spreading it out into a spectrum. Photographs taken with 

 the hydrogen lines give dark structures scattered all over 

 the sun. In many cases, especially away from simspots 

 and other distui-bed regions, these dark hydrogen regions 



correspond closely m form with the calcium flocculi. But 

 in the neighbourhood of sunspots there are frequently 

 very marked differences of an interesting kind. In 

 much disturbed regions bright hydrogen structures are 

 occasionally shown. These usually correspond with very 

 liright calcium flocculi shown in 

 high level pictures, but it is a sig- 

 niticant fact that no evidence of 

 such increased brightness is notice- 

 able in the low level calcium j)ic- 

 tures taken at the same time. 

 Through a comparison with the 

 hydrogen photographs we have 

 found that dark calcium flocculi 

 are occasionally present. I shall 

 give a brief preliminary discussion 

 of these results in a paper which I 

 liope to publish very soon. 



" It seems to me desirable to 

 point out that the calcium flocculi 

 ure not prominences, but are com- 

 ]>aratively low level phenomena in 

 the chromosphere. In the case of 

 eruptions, very brilliant pro- 

 minences are of course sometimes- 

 ^hown projected against the disk, 

 but these differ decidedly from 

 t he flocculi ordinarily photographed. 

 I am not yet in a position to state 

 to what level the dark hydrogen 

 clouds correspond. It is also im- 

 I'ortant to remark that attempts 

 to photograph the sun's disk 

 through the dark lines will give 

 very misleading results if low dis- 

 persion is employed. It is ab- 

 solutely necessary that the dis- 

 persion be so great as to produce 

 dark lines which are distinctly 

 wider than the second slit of the 

 spectroheliograph. Otherwise only 

 the ordinary faculse, which produce 

 increased brightness of the continuous spectrum, will be 

 shown on the plate." 



Hf tttrs. 



[Hie Editors do not bold themselves responsible for the opinions 

 or statements of correspondents.] 



BOREELLT'S COMET, 1903. 



TO THE EDITOKS OF KNOWLEDGE. 



Sirs, — The accompanying photographs were taken with 

 a reflector of 65 inches aperture and 28 inches focus, fixed 

 on to an 8.j inch equatorial reflector. 



The nucleus of the comet was kept as nearly central as 

 possible in the corner of a thick cross-wire micrometer 

 eyepiece, by setting the clock to drive at the rate of the 

 comet's motion in K.A., the decUnation being regulated by 

 hand. 



Cadett's Lightning Plates were used, with dilute Rodiual 

 and prolonged development, according to the method 

 described bv Mr. Eitchey at a recent E.A..S. meeting. 



No. 1, taken on 22nd" July, at lOh. 58m. to llh. .30m., 

 G.M.T. ; No. 2, taken on 26th Julv, at lOh. SUm. to 

 llh. Cm., G.M.T. 



A consideral)le change is shown during this interval of 

 nearly four days in the structure of the tail, and it is also 



