October, 1903.] 



KNOWLEDGE. 



229 



substances from the Orion stars may possibly be accounted 

 for. 



The argument for the identity of ccinipositiou with other 

 groups of stars is based on the continuity of the spectral 

 phenomena. Just as in passing from stars of the second 

 to stars of the first type the ordinary metallic lines give 

 way to the enhanced lines, so in proceeding onwards to 

 the Orion stars these lines are themselves displaced by- 

 lines chiefly due to gases. The changes are never abrupt, 

 and it is this unbroken sequence which leads to the belief 

 that the different types of stellar spectra indicate different 

 stages in the evolution of similar masses of matter. 



The names of st^mie of the brighter Orion stars, with the 

 groups to which they belong in Lockyer's classification, are 

 given in the following table : — 



focal lengths of about 33 inches. After pa«^tig through 

 the collimator lens the light faJl*, up6n 9, plane mirror, 

 from which it is reflected to ftl*ain of iwo'60' prisma, set at 

 minimum deviation for the K-line. A sejCond slit is placed 

 close to the focus of the camera leas>iaai3 m the case of the 

 photograph in questioUj.jwaSi Set at the qentre of the 

 K-liue. The great -itBfJ^iricfis telescoM ig'HSade to move 

 slowly in declination by meajis <>£« 'slow-motion electric 

 motor, and the sun's image consequently moves at a 

 uniform rate across the first slit. The photographic plate 

 is at the same time driven at the same rate across the 

 second slit by means of a shaft led down the tube of the 

 telescope from the motor. In the accompanying photo- 

 graph a double Hooke's joint is shown connecting this 

 shaft with the end of the screw which drives the plate, but 



It will be observed that the Orion stars are by no means 

 restricted to the constellation of Orion, except in the case 

 of the Alnitamian group, the brightest memi)ers of which 

 are the three stars forming the bslt of Orion. The name 

 Alnitam (often erroneously spelt Alnilam) in fact means 

 " a belt of spheres or pearls."* 



A study of the distribution of these stars on the celestial 

 sphere, and of their proper motions, has proved most 

 instructive. It is found that the great majority of them 

 lie near the ])lane of the Milky Way, and their vast 

 distances are indicated by the almost invariably small 

 proper motions, so that the two results together seem to 

 suggest that the Orion stars form a ring of almost 

 inconceival)le magnitude. 



THE SUN AS PHOTOGRAPHED ON THE 

 K-LINE. 



In Knowledge for January, 18'.'-t, Prof. U. E. Hale 

 publislie<l two photographs of the sun, which he had taken 

 ui nionocliroinatic light by means of the spcctroheliograph, 

 using for this purpose the K-line of calcium. We are 

 now able to ]>lace before the readers of Xnowledge, 

 again by the kindness of Prof. Hale, a recent photograph 

 of the sun ill K-liglit, which, by its groat b.^auty and 

 sharpness of detail, afford.s an index of the progress which 

 has been made iu perfecting the iiistnuiient. and the 

 method of work. 



The Kumfonl sii(!i(roheliograpli, with which the photo- 

 gra|)h was taken, is attai'hed to the great -tO-iueh refractor 

 of the Yerkes Observatory. As the focal length of that 

 telescope is 64 feet, the imago of the sun in the principal 

 focus is slightly over 7 inches in diameter. The objectives 

 (portrait lenses) of the collimator and camera of the 



• Lockjer, Jloy. iSoc. Proc, vol. 65, p. 187. 



spectroheliograph are each 6i inches in aperture, with 

 this has now been replaced by a belt connecting grooved 

 pulleys on the ends of the shaft and the screw respectively. 

 The two slits are each 8 inches in length, and are given the 

 proper curvature necessary to eliminate distortion of the 

 solar image. But the aperture of the spectroheliograph is 

 not quite sufiicient for a 7-inch image of the sun, and this 

 occasions much loss of light at both ends of the long slit, 

 and is the cause of the falling off in brightness at the two 

 opposite limbs of the sun, which will be noticed in the plate. 

 A comparison of the photograph on the plate with an 

 ordinary photograph of the sun, taken on the same day 

 with the Thompson photoheliograph of the Royal 

 Oliservatory, Greenwich, shows three very striking 

 differences between them. The " mottling " on the K-line 

 jihotograph is far more pronounced, and its reticulations 

 display a more open mesh than the " white-light " photo- 

 graph taken at Greenwich. Next, the bright calcium 

 clouds, of which five principal groups are seen in the jjlate, 

 though they correspond in position to the groups of 

 facula; shown on the Greenwich photograph, and even to a 

 very considerable extent iu form, are more extensive 

 and, especially near the centre of the sun, far more 

 distinct. Prof. Hale proposes, for the sake of clearness, to 

 give these calcium clouds a specific name, and suggests 

 the name " Flocculi " for them. Thirdly, the dark spots 

 cover a much greater area on the Greenwich photograph 

 than on that taken by the Yerkes sport lolieliograph. This 

 is due to the way in which these bright calcium Flocculi 

 extend themselves above the spots, concealing the greater 

 part of each group. The three principal spot-groups are 

 the same as those shown in the plate of the June number 

 of Knowi.eihik, which is a reproduction of a photograph 

 taken three days later than the present one. The great 

 calcium cloud halfway between the sun's centre and the 

 north limb, iu the present plate, corresponds to the spot- 



