494 Prof. J. Lovering on the Mathematical and 



light, in the wave-length through the prism, and in the refraction, 

 was compensated by the physiological aberration when the rays 

 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 mathematicians might be enumerated who have 

 taken part in it. FresnePs explanation has encountered diffi- 

 culties 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 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 neighbourhood of the place in which 

 astronomers have put the solar apex are moving, apparently, to- 

 ward the earth, while those in the opposite part of the sky re- 

 cede. 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 awaken no dis- 

 trust 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 remove the discrepancy by throwing discredit 

 on the estimates of star-distances made independently by Struve 

 and Argelander from different lines of thought. 



Next we ask if it is certain that even the motion of the lumi- 

 nary will change the true wave-length, the period of oscillation, 

 and the refrangibility 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 



