ADDRESS OF PROFESSOR LOVERING. 645 
emerged. Very recently, Ketteler of Bonn has gone over the 
whole ground again with great care, studying not only Arago’s 
case but the general one, in which the direction of the light made 
any angle with the motion of the earth: and he proves that the 
light will always enter the eye in the same apparent direction as 
it would have done if the earth were at rest. The mathematical 
and physical view taken of this subject by Fresnel, has been under 
discussion for sixty years, and forty eminent physicists and math- 
ematicians might be enumerated who have taken part in it. Fres- 
nel’s explanation has encountered difficulties and objections. Still, 
it is consistent not only with Arago’s negative result but with 
the experiments on diffraction by Fizeau and Babinet, and the 
preponderance of mathematical evidence is on that side. Mr. 
Huggins runs counter to the general drift of physical and algebrai- 
cal testimony (although he appears to be sustained by the high 
authority of Maxwell), when he attributes some displacement of 
the spectrum lines to the motion of the earth, and qualifies the 
observed displacement on that account. The number of stars 
Which Huggins has observed is insufficient for any sWeeping gen- 
eralization. And yet he seems inclined to explain the revelations 
of his spectroscope, not by the motion of thé stars, but by that of 
the solar system: because those stars which are in the neighbor- 
hood of the place in which astronomers have put the solar apex 
are moving, apparently, towards the earth, while those in the op- 
Posite part of the sky recede. If it be true that the earth’s annual 
Motion produces no displacement in the spectrum, then the motion 
of the solar system produces none. Or, waiving this objection, if 
the correct explanation has been given by Huggins, astronomers 
have failed, by their geometrical method, of rising to the full 
Magnitude of the sun’s motion. The discrepancy appears to 
_ *Waken no distrust in Mr. Huggins’ mind as to the delicacy of the 
‘“Pectrum analysis or the mathematical basis of his reasoning. 
On the contrary, he would remove the diserepancy by throwing 
a redit on the estimate of star-distances made independently by 
ve and Argelander from different lines of thought. 
canes we ask, if it is certain that even the motion of the lami- 
ywn change the true wave-length, the period of oscillation, 
ANd the refrangibility, of the light which issues from it. The 
TR ly received opinion on this subject has not been allowed 
OOS a Unchallenged. It is fortified by more than one analogy : 
