Jtily 1 8, 1872] 



NATURE 



233 



piece 2, the lines appear to be coincident with those of liydrogen. 

 In consequence of tlie uncertainty of the character ol the first 

 line, which is single, while that of nitrogen is double, this deter- 

 mination can now only be made by means of the comparison of 

 the third line with that of hydrogen. This third line becomes 

 very faint from the great loss of light unavoidable in a spectro- 

 scope that gives a sufficient dispersive power, and the comparison 

 can only be attempted when the sky is very clear and the nebula 

 near the meridian. 



2. 0)1 the Motion of seme S/iWS tivanfs or from the Earth 



In the early part of 1S6S I had the honour of presenting to 

 the Royal Society some observations on a small change of re- 

 frangibility which I had observed in a line in the spectrum of 

 Sirius as compared with a line of hydrogen, from which it 

 appeared that the star was moving from the earth with a velocity 

 of about twenty-five miles per second, if the probable advance of 

 the sun in space be taken into account.* 



It is only within the last few months that I have found myself 

 in possession of the necessary instrumental means to resume this 

 inquiry, and since this time the prevalence of bad weather has 

 left but few nights sufficiently fine for these delicate observati<ras. 



Some time was occupied in obtaining a perfectly trustworthy 

 method of comparison of the spectra of siars with those of 

 terrestrial substances, and it was not until I had arranged the 

 spark withm the tube, as described at the beginning of this note, 

 that I felt confidence in the results of my observations. 



It may be well to state some circumstances connected with 

 these comparisons which necessarily make the numerical estima- 

 tions given further on less accurate than I could wish. Even 

 when spectroscope C, containing four compound prisms, and a 

 magnifying power of 16 diameters are used, the amount of the 

 change of refrangibility to be observed appears very small. The 

 probable error 01 these estimations is therefore large, as a shift 

 corresf onding to five miles per second (about ^V of the distance 

 of D' to D-), or even a somewhat greater velocity, could not be 

 certainly observed. The difficulty arising from the apparent 

 smallness of the change of refrangibility is greatly increased by 

 some other circumstances. The star's light is faint when a 

 narrow slit is used, and the lines, except on very fine nights, 

 cannot be steadily seen, in consequence of the movements in our 

 atmosphere. Further, when the slit is narrow, the clock's 

 motion is not uniform enough to keep the spectrum steadily in 

 view ; for these reasons I found it necessary to adopt the method 

 of estimation by comparing the shift with a wire of knoHU 

 thickness, or witii the interval between a pair of close lines. I 

 found that, under the circumstances, the use of a micrometer 

 would have given the appearance only of greater accuracy. I 

 wish it therefore to be understood that I regard the following 

 estimations as provisional, as I hope, by means of apparatus now 

 being constructed, to be able to get more accurate determina- 

 tions of the velocity of rhe motions. 



Sirius. — The comparison of the line at F with the correspond- 

 ing line of hydrogen was made on several nights from January 18 

 to March 5. Spectroscope C and eye-pieces 2 and 3 were used. 

 These observations confirm the conclusion arrived at in my 

 former paper, that the star is moving from the earth ; but they 

 ascribe to the star a velocity smaller than that which I then 

 obtained. 



These observations on different days show a change of re- 

 frangibility corresponding to a velocity of from twenty-six miles 



Februar und Marz 1872, fand ich wiederum, so wie es Huggins frulier ge- 

 sehen fiat, unzweifelfiaft (2) > (3). Die relative Intensitiit der drei Lichlarten 

 habe ich mehrfach in Zahlen geschatzt und erhielt z. B. in den letzten 



Nil 



Ma 



M.H 



(■) 



(2) 58 63 



(3J 49 52 



• Phil. Trans. 1868, pp. 529-550. As a curious instance in which later 

 methods of observations liave been partially anticipated, a reference may be 

 made to an ingenious paper in the Philosophical Transactions for 1783, vol. 

 Ixxiv., by the Kev. John Mitchell, entitled "On the means of discovering 

 the Distance, Magnitude, &c., of the Fi.\ed Stars, in consequence of the 

 Diminution of the Velocity of their Light." The author suggests that by the 

 use of a prism " we might be able to discover diminutions in the velocity of 

 light as perhaps a hundredth, a two hundredth, a five hundredth, one even 

 thousandth part of the whole." Lut he then goes on to reason on the pro- 

 duction of this diminished velocity by the attraction produced on the material 

 particles of light by the matter of the stars, and that the diminutions stated 

 above would be '' occasioned by spheres whose diameter should be to the 

 sun, provided they were of the same density, in the several proportions of 70, 

 50, 3o, and to 22 to t respectively." 



to thirty-six miles per second. She part of the earth's orbital 

 motion from the star varied on these days from ten miles to four- 

 teen miles per second. We may take, therefore, eighteen to 

 t\\'enty-t\\'o miles per second as due to the star. 



The difference of this estimate, which is probably below rather 

 than in excess of the true current from that which I formerly 

 made, may be due in part or entirely to the less perfect instru- 

 ments then at my command. At the same time, if Sirius be 

 moving in an elliptic orbit, as suggested by Dr. Peters, that part 

 of the star's proper motion, which is then in the direction of the 

 vesical ray, would constantly vary.* 



Betelgeiix (a Orionis). — In the early observations of Dr. Miller 

 and myself on this star, we found that there are no strorg lines 

 coincident with the hydrogen lines at C and I*". The line H a 

 falls on the less refrangible side of a group of strong lines, and 

 H ^ occurs in the space between two groups of strong lines, 

 where the lines are faint. On one night of unusual steadiness in the 

 air, when the finer lines in the star's spectrum were seen with 

 more than ordinary distinctness, I was able with the more power- 

 ful instruments now at my command to see a nan'ow defined Hue 

 in the red apparently coincident with H a, and a similar line at 

 the position of H /3. These Tnes are much less intense than the 

 lines C and F in the solar spectrum ; there are certainly no bright 

 lines in the star's spectrum at these places. 



The most suitable lines in this star for comparison with terres- 

 trial substances for ascertaining the star's motion are the lines of 

 sodium and of magnesium. The double character of the one 

 line agreeing exactly with that of sodium, and the further cir- 

 cumstance that the more refrangible of the lines is the stronger 

 one, as is the case in spectrum of sodium and in the solar spectrum, 

 and the relative distances from each other and comparative 

 brightness of the three lines, which correspond precisely to the 

 triple group of magnesium, can allow of doubt that these lines 

 in the star are really produced by the vapours of these substances 

 existing there, and that we may therefore safely take any small 

 displacement of either set of lines to show a motion of the star 

 towards or from the earth. The lines due to sodium are perhaps 

 more intense, but are as narrow and defined as the lines Dj, D.^ in 

 the solar spectrum ; they fall, however, within a group ol very 

 fine lines ; this circumstance may possibly account for the 

 nebulous character which has been ass gned to them by Father 

 Secchi. 



The bright lines of sodium were compared with spectroscope 

 B and eye-piece 3 ; they appeared to fall very slightly above the 

 pair in the star, showing that the stellar lines had been degraded 

 by the star's motion from the earth. The amount of displace- 

 ment was estimated at about one-fifth of the distance of D^ from 

 Do, which is probably rather smaller than the true amount. This 

 estimation would give a velocity of separation of thirty-seven 

 miles per second. At the time of observation the. earth was 

 moving from the star at about fifteen miles per second, leaving 

 twenty-two miles to be due to the star. 



When magnesium was compared, a shift in the elevation, and 

 corresponding in extent to about the same velocity of recession, 

 was observed ; but in consequence of other lines in the star at 

 this place, the former estimation, based on the displacement of 

 the lines of sodium, was considered to be more satisfactory, t 



Rigel. — The lines of hydrogen are strong in the spectrum of 

 this star, and are suitable for comparison. 



The line of H /3 is not so broad as it appears in the speculum 

 of Sirius, but is stronger than F in the solar spectrum : this line 

 was compared by means of spectroscope C and eye-pieces 2 and 

 3. The line of terrestrial hydrogen falls above the middle of the 

 line in the star ; the star is therefore receding from the earth. The 

 velocity of recession may be estimated as rather smaller than 

 Sirius, probably about thirty miles per second, the earth at the 

 time of observation moving from the star with a velocity of 

 fifteen miles, leaving about fifteen miles as due to the star. 

 This estimate is probably rather smaller than the true velocity 

 of the star. 



* H. Vugel at Bothkamp seems to have repeated mv observations on 

 Sirius with the necessary care. He says (Asiron. Nachr. No. 1S64) ; — 

 " Mit der eben beschriebenen Anordnung gelang es Herrn Dr. Lohse und 

 mir am 22 Milrz .1S71] bei ganz vorziiglicher Luft die Nichtcoincidenz der 

 drei Wasserstofflinien H a, H /J, und H 7, der Geissler'schen Rohre mit 



den entsprechenden Linien des Siriusspectrums zu sehen mit Beriick- 



sichtigung der Geschwindigkeit der Erde zur Zeit der Beobachtung berechnet 

 sich die Geschwindigkeit mit welcher sich Sirius von der Erde bewegt zu 

 io"o Meilen in der Secunde, wogegen Procyon sich 138 Meilen in der 

 Sccunde von unserer Erde entfernen wiirde." 



t I had the pleasure on one evening of showing the displacement of the 

 lines in Sirius and a Orionis to Mr. Christie, First Aisistant at the Greenwich 

 Observatory. 



