PROFESSOR LOVERING'S ADDRESS. 309 



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 dis- 

 cussion for sixty years, and forty eminent physicists and mathe- 

 maticians might be enumerated who have taken part in it. Fresnel's 

 explanation has encountered difficulties and objections. Still, it is 

 consistent not only with Arago's negative result, but with the experi- 

 ments 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 algebraical 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 generalization. And yet he seems inclined to explain 

 the revelations of his spectroscope, not by the motion of the stars, but 

 by that of the solar system ; because those stars which are in the 

 neighborhood of the place in which astronomers have put the solar 

 apex are moving, apparently, toward 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 cor- 

 rect 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 awaken no distrust in Mr. 

 Huggins's mind as to the delicacy of the spectrum analysis or the 

 mathematical basis of his reasoning. On the contrary, he would re- 

 move the discrepancy by throwing discredit on the estimate of star- 

 distances made independently by Struve and Argelander from differ- 

 ent lines of thought. 



Next, we ask, if it is certain that even the motion of the luminary 

 will change the true wave-length, the period of oscillation, and the re- 

 frangibility, of the light which issues from it. The commonly-received 

 opinion on this subject has not been allowed to pass unchallenged. It 

 is fortified by more than one analogy ; but it is said that comparison 

 is not always a reason. It is not denied that, when the sonorous body 

 is approaching, the sound-waves are shortened, the number of impulses 

 on the ear by the condensed air is increased, and the pitch of the sound 

 is raised. Possibly, the color of light would follow the same law ; but 

 there is no experiment to prove it, and very little analogy exists be- 

 tween the eye and the ear. There is no analogy, whatever, between 

 the subjective sensation by either organ and the physical action of 

 the prism. The questions at issue are these : Does refraction depend 



