154 REPORT—1868. 
of the stars relatively to our system. The great velocity of light relatively to 
the known planetary velocities and to the probable motions of the few stars 
of which the parallax is known, showed that any alterations of position which 
might be expected from this cause in the lines of the stellar spectra would 
not exceed a fraction of the interval between the double line D, a change 
which could not be detected in our instrument. We were, however, in pos- 
session of the information that the stars, the spectra of which had been com- 
pared with the necessary care, were not moving towards or from the earth 
with a velocity so great as 190 miles per second. Among these stars are 
Aldebaran, a Orionis, ( Pegasi, Sirius, a Lyre, Capella, Arcturus, Pollux, 
and Castor. 
Recently (in 1866) Klinkerfues published a memoir on the influence of the 
motion of a source of light upon the refrangibility of its rays, and described 
therein a series of observations, from which he deduces certain amounts of 
motion in the case of some objects observed by him. As Klinkerfues em- 
ploys an achromatic prism, it does not seem possible by his method to obtain 
any information of the motions of the stars ; for in such a prism the difference 
of period of the luminous waves would be as far as possible annulled. It is, 
however, conceivable that his observations of the light when travelling from 
E. to W., and from W.:to E., might show a difference in the two cases, 
arising from the earth’s motion through the ether*. 
Father Secchi has quite recently called attention to this subject. In his 
paper he states that he has not been able to detect any change of refrangi- 
bility in the case of certain stars of an amount equal to the difference 
between the components of the double line D. These results are in accord- 
ance with those obtained by Dr. Miller and myself in 1863, so far as they 
refer to the stars whith had been examined by us. Father Secchi’s method 
of using an unrefracted image as a fiducial mark, with diverging rays pass- 
ing through the prisms, might, it is conceivable, be open to objection. He 
appears to consider that, to produce a certain alteration of refrangibility, half 
the velocity would be required in the case of the approach of a star, of that 
which would be necessary if the star were receding. ‘This is not the caso; 
for equal velocities of separation or approach give equal change of wave- 
length. It is true that a difference of an octave is produced by a relative 
velocity of separation equal to that of light, and by a velocity of approach 
equal to half that of light; but the difference in length of a wave and its 
octave below (which is twice as long) is, in the same proportion, greater than 
the difference between it and the octave above (which is half as long) +. 
The subject of the influence of the motions of the heavenly bodies on the 
index of refraction of light had already (in 1864) occupied the attention of 
Mr. Maxwell, F.R.S8., who made some experiments in an analogous direction. 
In the spring of last year Mr. Maxwell sent to me a statement of his views 
and experiments, which he has permitted me to include in a paper pre- 
sented by me to the Royal Society. 
Mr. Maxwell puts the subject in the following way :— 
* Let a source of light be such that it produces 2 disturbances or vibra- 
tions per second, and let it be at such a distance from the earth that the 
light requires a time T to reach the earth. Let the distance of the source 
of light from the earth be altered, either by the motion of the source of light, 
* “Fernere Mittheilungen iiber den Hinfluss der Bewegung der Lichtquelle auf die 
Beep ait eines Strabls, von W. Klinkerfues,” Nachr. der K. G. der W: zu Gottingen, 
No, 4, s. 33. 
T Comptes Rendus, 2 Mars 1868, p 398. ¢ Phil. Trans. 1868. 
