58 Royal Society ; — 



It is also shown that at the time of the observations this nebula 

 was not receding from us with a velocity greater than 10 miles per 

 second ; for this motion, added to the earth's orbital velocity, would 

 have caused a want of coincidence of the lines that could have been 

 observed. If the nebula were approaching our system, its velocity 

 might be as much as 20 or 25 miles per second ; for part of it3 

 motion of approach would be masked by the effect of the motion of 

 the earth in the contrary direction. 



§ IV. Observations of Stars. 



A detailed description is given of the comparisons of the line in 

 Sirius corresponding to F, with a line of the hydrogen spectrum, 

 and of the various precautious which were taken against error in 

 this difficult and very delicate inquiry. The conclusions arrived at 

 are : — that the substance in Sirius which produces the strong lines 

 in the spectrum of that star is really hydrogen ; further, that the 

 aggregate result of the motions of the star and the earth in space, at 

 the time the observations were made, was to degrade the refrangibi- 

 lity of the dark line in Sirius by an amount of wave-length equal to 

 0*109 millionth of a millimetre. 



If the velocity of light be taken at 185,000 miles per second, and 

 the wave-length of F at 486*50 millionths of a millimetre, the ob- 

 served alteration in period of the line in Sirius will indicate a mo- 

 tion of recession between the earth and the star of 4 T4 miles per 

 second. 



At the time of observation, that part of the earth's motion which 

 was in the direction of the visual ray was equal to a velocity of about 

 12 miles per second from the star. 



There remains unaccounted for a motion of recession from the 

 earth amounting to 29 '4 miles 'per secondly which we appear to be 

 entitled to attribute to Sirius. 



Reference is made to the inequalities in the proper motion of 

 Sirius ; and it is stated that at the present time the proper motion 

 of Sirius in declination is less than its average amount by nearly the 

 whole of that part of it which is variable, which circumstance may 

 show that a part of the motion of the star is now in the direction of 

 the visual ray. 



Independently of the variable part of its proper motion, the whole 

 of the motion which can be directly observed by us is only that 

 portion of its real motion which is at right angles to the visual ray. 

 Now it is precisely the other portion of it, which we could scarcely 

 hope to learn from ordinary observations, which is revealed to us by 

 prismatic observations. By combining both methods of research, it 

 may be possible to obtain some knowledge of the real motions of the 

 brighter stars and nebulae. 



Observations and comparisons, similar to those on Sirius, have 

 been made on a Canis Minoris, Castor, Betelgeux, Aldebaran, and 

 some other stars. The author reserves the results until these ob- 

 jects have been reexamined. It is but seldom that the atmosphere 



