234 



NATURE 



\yuly 18, 1872 



Castor. — The spectra of the two component stars of this 

 double star blend in the spectroscope into one spectrum. Tlie 

 line H ;3 is rather broad, nearly as much so as tlie same line in 

 the spectrum of Sirius. 



The narrow line of rarefied hydrogen was compared in spec- 

 troscope B with eye-piece 3 ; it appeared to fall on the more re- 

 frangible side of the middle of the line in the star, leaving more 

 of the dark line on tlie side towards the red. The shift seemed 

 to be rather greater than that in Sirius, and may probably \r^ 

 taken at from 40 to 45 miles per second ; but the earth's orbital 

 motion was nearly 17 from the star, thus leaving about 25 niilei 

 for the apparent velocity of tlie star. This result rests at pre>ent 

 on observations on one night only, but they ^eemed at the time 

 to be satisfactory. 



Ri-gulus. — The line at F rather broad. Tlie corresponding 

 line of hydrogen falls on the more refrangible side of the middle 

 of the dark line in the star. The air was unfavourable on all the 

 evenings of comparison ; a nough estimate gives a velocity oi 

 from 12 to 17 miles for the velocity of recession between the s'af 

 and the sun. 



3 ami S Lconis. — These stars were compared with hydrogen ; 

 they appear to be moving from the earth, but the want of steadi- 

 ness in the air prevented me from mailing a satisfactory estimate 

 of their velocity. I suspected their motion to be smaller than 

 that of Regulus. 



/3> 7, 5, 6, f Ursd- Majivis. — All these stars have similar 

 spectra, in which the line F is strong, though there are small 

 differences in the breadth of the line. They were compared 

 with hydrogen, and appear to be moving from our system with 

 about the same velocity. Probably their motion may be taken 

 to be not far from 30 miles per second. The earth's motion at 

 the time of observation was from 9 miles to 13 miles for these 

 stars, leaving a probable velocity of recession of 17 to 20 miles 

 per second. In the case of the double star f, the spectrum con- 

 sisted of the light of both stars. 



7) Ursa Majoris was also compared with hydrogen. I believe 

 it shows a motion from the earth, but the observations of this star 

 are at present less satisfactory. 



a Virgin'is attd a Corontc BorcaUs. — These stars were com- 

 pared with hydrogen. I suspect that they are receding, but I 

 have not had nights sufl'iciently fine to enable me to make satis- 

 factory observations of these stars. 



In addition to these stars some observations (which are less 

 satisfactory on account of the unfavourable state of the weather 

 at the time) appear to show that the stars Procyon, Capella, and 

 possibly Aldebaran, are moving from the earth. 



The stars which follow have a motion of approach. 



Arclurus. — In the spectrum of this star the linos of hydrogen, 

 of magnesium, and of sodium are sufficiently distinct for com- 

 parison. I found the comparison could be most satisfactorily 

 nude with magnesium. 



The bright lines of magnesium fall on'the less refrangible side 

 of the corresponding dark lines in the star's spectrum, showing 

 that the star is approaching the earth. I estimated the shift at 

 about i to J of the interval between Mg., and Mg.j ; this amount 

 of displacement would indicate a velocity of approach of 50 

 miles per second. To this velocity must be added the earth's 

 orbital motion from the star of 5 '25 miles per second, increasing 

 the star's motion to 55 miles per second. 



When I can get favouralde weather, I hope to obtain inde- 

 pendent estimations from the lines of sodium and of hydrogen. 



o Lyric. — In the spectrum of Vega the line corresponding to 

 H ^ is strong and broad. Comparisons were made on several 

 nights, but on one evening only was the air favourable. The 

 observations are accordant in showing that the narrow bright 

 line from a Geissler's tube falls on the less refrangible side of the 

 middle of the line in the star, thus leaving more of the line on 

 the side towards the violet. The estimations give a motion of 

 approach between the earth and the star of from 40 to 50 miles per 

 second, to which must be added 3*9 miles after theearth's motion 

 from the star. 



a Cygni. — The hydrogen line at F in the spectrum of this star 

 is narrower than in the spectrum of Sirius and of a Lyrje, though 

 probably rather broader than the same line in the solar spectrum. 

 I have at present observations made on two evenings only, on 

 both of which tlie state of the air was unfavourable, of the com- 

 parison of this line with that of terrestrial hydrogen. They 

 give to the star a motion of approach of about 30 miles per 

 second, which would have to be increased by 9 miles, the velocity 

 at the time of the earth from the star. 



Pollux. — The lines of magnesium and those of sodium are 

 very distinct in the spectrum of this star. As the air was not 

 very steady at the time of my observations, I found it more satis- 

 factory to use for comparison the lines of magnesium, which are 

 rather stronger than those of sodium. The three lines of magne- 

 sium appeared to be less refrangible than the corresponding 

 dark lines in the spectrum of the star by about one-sixth of the 

 interval from Mgo to Mg.j. This estimation would represent a 

 velocity of approach equal to about 32 mdes per second. The 

 earth's motion from the star was 1 7 '5 miles, which increases tlie 

 apparent velocity of approach to 49 miles per second. On one 

 evening only was the air favourable enough for a numerical 

 estimate, but the observations were entered in my observatory- 

 book as very satisfactory. 



6. Ursie Majoris. — The spectrum of this star is very different 

 from the spectra of the other bright stars of this constellation. 

 The line at F is not so strong, while the lines at b are more 

 distinct, and are sufficiently strong for comparison with the 

 bright lines of magnesium. The bright lines of this metal fall 

 on the less rerrangible side of the dark lines, and show a motion 

 of approach of from 35 to 50 miles per second. The earth's 

 m ition of n '8 miles from the star must be allowed for. 



7 Leonis and e Booth. — In both these double stars the com- 

 pared spectrum due to the light of both important stars were 

 observed. Both stars are most conveniently compared with mag- 

 nesium. I do not consider my observations of these stars as 

 quite satisfactory, but they seem to show a movement of ap- 

 proach ; but further observations are desirable. 



The stars 7 Cygiii, a Pigasi, 7 Pigasi, and a Andronicdic were 

 compared with hydrogen on one night only. It is probable that 

 these stars are approaching the earth, but I wish to re-observe 

 them before any numerical estimate is given of their motion. 



7 Cassiopchc — (Jn two nights I compared the bright lines which 

 are present in its spectrum at C and F with the bright lines of 

 terrestrial hydrogen. The coincidence appeared nearly perfect 

 in spectroscope C with eye-pieces 2 and 3 ; but on the night of 

 least defini'ion I suspected a minute displacement of the bright 

 line towards the red when compared with H 3, As the earth's 

 orbital motion from the star at the time was very small, about' 

 3'25 miles per second, which corresponds to a shift that could 

 not be detected in the spectroscope, it seems probable that 

 7 Cassiopei.Te has a small motion of recession. 



In the calculation of the estimated velocities the wavedengths 

 employed are those given by Angstrom in his " Recherches sur 

 le spectre solaire," Upsal, 186S. The velocity of light was 

 taken at 185,000 miles per second. 



"™lThe velocities of approach and of recession which have been 

 assigned to the stars in this paper represent the whole of the 

 motion in the line of sight which exists between them and the 

 sun. As we know that the sun is moving in space, a certain 

 part of these observed velocities must be due to the solar motion. 

 I have not attempted to make this correction, because, though 

 the direction of the sun's motion seems to be satisfactorily ascer- 

 tained, anyestimxte that can be made at present of the actual 

 velocity with which he is advancing must rest upon suppositions, 

 more or less arbitrary, of the average distance of stars of different 

 magnitudes. It seems not improbable that this part of the stars' 

 motions may be larger than would result from Otto Struve's 

 calculations, which give, on the supposition that the average 

 p.arallax of a star of the first magnitude is equal to o"'209, a 

 velocity but little greater than one-fourth of the earth's annual 

 motion in its orbit. 



It will be observed that, speaking generally, the stars which 

 the spectroscope shows to be moving from the evrth (Sirius, 

 Betelgeux, Rigel, Procyon) are situated in a part of the heavens 

 opposite to Hercules, towards which the sun is advancing ; while 

 the stars in the neighbourhood of this region, as Arcturus, Vega, 

 a Cygni, show a motion of approach. There are in the stars 

 already observed exceptions to this general statement ; and there 

 are some other considerations which appear to show that the sun's 

 motion in space is not the only, or even in all cases, as it may be 

 found, the chief cause of the observed proper motions of the 

 stars. 



There can be little doubt but that in the observed stellar move- 

 ments we have to do with two other independent motions, 

 namely, a movement common to certain groups of stars, and also 

 a motion peculiar to each star. 



Mr. Proctor has brought to light strong evidence in favour of 

 the drift of stars in groups having a community of motion, by his 

 graphical investigation of the proper motions of all the stars in 



