T08 



NA TURE 



[March 25, 1909 



OVR ASTRONOMICAL COLUMN. 



PnoTOGRrtnis OF MoKEiiousii's Comet, igoSt. — Four 

 excellent photographs showing remarkable details in the 

 structure of comet 19081; are reproduced on two plates 

 accompanying Circular No. 148 of the Harvard College 

 Observatory. 



These photographs are selected from a series of fifty- 

 three taken, between September 3 and November 29, 

 by the Rev. Joel Metcalf at Taunton, Mass. The instru- 

 ments employed were two photographic doublets, one of 

 12 inches aperture and 87-5 inches focal length, the other 

 of 5-8 inches aperture and 20 inches focal length, both 

 constructed by the observer. 



.'\s the nucleus of this comet was too indefinite to be 

 " followed " successfully, Mr. Metcalf employed the method 

 by which he has obtained such remarkable success in the 

 photography of minor planets. This consisted in following 

 on an adjacent star and moving the cross-wires, with a 



Comet MorehDUse(i9o3<:), 190S No 



L— iih.4Sm.(G.M.T.) 



micrometer screw, every minute by an amount sufificient to 

 compensate for the comet's theoretical motion as indicated 

 by the ephemeris. 



The photographs are reproduced in half-tone from double- 

 contact prints, thus intensifying the fainter details of the 

 tail, although some of the finer structure of the more 

 exposed head has been lost in the process. On the photo- 

 graph of 1908 November 15, iih. 6m. (G.M.T.), the main 

 tail presents a twisted appearance more marked than on 

 any other photograph we have yet seen. The second photo- 

 graph shows a remarkable waviness of the stronger 

 northern edge of the tail with curious interlacings, and, as 

 seen from the configuration of the surrounding stars, it is 

 a connecting link between the November 16 and 18 photo- 

 graphs reproduced by Prof. Barnard in the January number 

 of the Aslropliysical Journal. 



NO. 2056, VOL. 80] 



The photograph which we here reproduce was taken with 

 76 minutes' exposure on November 21, the time of mid- 

 exposure being iih. lom. (G.M.T.). It will be noticed 

 that, in addition to that contiguous to the nucleus, there 

 are two constrictions in the main tail, apparently indicating 

 two separate outbursts of activity on the part of the nucleus 

 in the ejection of tail matter ; the approximate position of 

 the centre of the plate is i8h. s8m., -1-1° 30'. 



For the benefit of other observers who wish to make a 

 detailed study of comet 1908c, Prof. Wolf publishes in 

 Astronomischc Nachrichten, No. 43 11, a list of the photo- 

 graphs taken with ten different objectives at the Heidel- 

 berg Observatory. 



Between September 6 and November 27, 1908, 147 plates 

 were taken on thirty-three different nights, and the present 

 list gives the date, time, and duration of each exposure, 

 with a note as to the instrument employed. 



Relation between the Magnitudes and Colours of 

 .Stars. — In No. 4312 of the Astronomische Nachrichten 

 (p. 249) Herren Muller and Kempf discuss the relationship 

 which holds between the magnitudes and colours of the 

 stars of the Potsdam Photometrischcn Durchmusterung. 



The number of stars included in the discussion is 14,172, 

 and these are tabulated, in tenths of a magnitude from 

 00 to 99, under four divisions of colour, viz. white, 

 yellowish-white, whitish-yellow, and yellow, the last- 

 named including the few orange and red stars. A summary 

 table shows that by far the greatest number (6324) of the 

 stars considered are classified as yellowish-white, a little 

 more than half this number are whitish-yellow, whilst the 

 " white" and the "yellow, &c.," stars are equal, 2043 in 

 each case. In another table, showing percentages, the 

 white stars show a tendency to increase as the fainter 

 stars are reached, and this increase is more marked in the 

 yellowish-white class. In the whitish-yellow class the per- 

 centage decreases in both directions from the seventh 

 magnitude, although the deficiency is more marked towards 

 the fainter stars. The most striking variation is in the 

 " yellow, &c.," class, w'here the percentages rapidly decrease 

 between magnitude 4-5 to magnitude 90. 



A second part of the discussion deals with the rela- 

 tion between colours and magnitudes and the galactic 

 latitudes of the stars. The results show, inter alia, that 

 the maximum of the brighter white stars occurs in galactic 

 latitudes —11° to —30°, whilst for the fainter white stars 

 the minimum is not at the galactic pole, but in galactic 

 latitudes 4-30° to 4-50°. 



A Remarkable Prominence. — No. 2, vol. xxxviii., of the 

 Meniorie della Societa degli Spettroscopisti Italiani con- 

 tains an account of two remarkable prominences observed 

 by Father Chevalier at the Z6-s& Observatory on July 30 

 and 31, 1908. 



Both prominences were observed in about position-angle 

 80°, and were evidently connected with the fine spot groups 

 which appeared round the limb at the beginning of August 

 last. Their changes of form and their general shapes are 

 shown by a series of drawings given on a plate accompany- 

 ing the paper, and it is seen that both formed well-marked 

 arches ; but it is to the spectrum observations that the 

 greatest interest is attached. 



On July 30 not only were C, D,, and F seen reversed, 

 but also the lines of helium at X\ 6678-2 and 70655 ; none 

 of the metallic lines was bright, but between b and F two 

 bright lines, probably helium \ 5016 and \ 4922, were 

 found. 



In addition to the bright lines, however, there was a 

 continuous spectrum, due to the prominences, strong 

 enough to efface, or weaken, the atmospheric spectrum on 

 which it was superposed. A similar phenomenon was 

 observed on .August 3, 1872, by Young, who attributed its 

 appearance to an abnormal pressure on the gases emitting 

 it. In the present case it is difficult to see how pressure 

 could operate, and Father Chevalier is inclined to attribute 

 the bright continuous spectrum to heated solid particles 

 condensed from the metallic vapours carried up by the rush 

 of gases. 



.\ strange bright line at about X 587250 was also seen 

 both on July 30 and 31, and on the latter date a similar, 

 but weaker, line was seen on the other side of Dj at about 

 A 5879-9. 



