644 ADDRESS OF PROFESSOR LOVERING. 
gratifying as it is to the imagination and the aspirations of science, 
the whole superstructure must fall. 
I am thus suddenly brought face to face, with the second head 
‘of my subject: the mathematical and philosophical state of the 
physical sciences. 
The luminiferous ether and the undulatory theory of light have 
always troubled what is supposed to be the imperturbable charac- 
ter of the mathematics. The proof of a theory is indisputable 
when it can predict consequences, and call successfully upon the 
observer to fulfil its prophecies. It is the boast of astronomers 
that the law of gravitation thus vindicates itself. The undulatory 
theory of light has shown a wonderful facility of adaptation to 
each new exigency in optics, and has opened the eye of observa- 
tion to see what might never have been discovered without the 
promptings of theory. But this doctrine, and that of gravitation 
-also, have more than once been arrested in their swift march an 
obliged to show their credentials. After Fresnel and Young had 
secured a firm foothold for Huyghens’ theory of light in mechanics 
and experiment, questions arose which have perplexed, if not 
baffled, the best mathematical skill. How is the ether affected by 
the gross matter which it invests and permeates? Does it move 
when they move? If not, does the relative motion between the 
ether and other matter change the length of the undulation or 
the time of oscillation? These queries cannot be satisfactorily 
answered by analogy, for analogy is in some respects wanting 
between the ether and any other substance. Astronomy says that 
aberration cannot be explained unless the ether is at rest. Optics 
replies that refraction cannot be explained unless the ether moves. 
Fresnel produced a reconciliation by a compromise. The ether 
moves with a fractional velocity large enough to satisfy refraction, 
but too small to disturb sensibly the astronomer’s aberration. In 
1814, Arago reported to Fresnel that he found no sensible differ- 
ence in the prismatic refraction of light, whether the earth ni 
moving with full speed towards a star or in the opposite direction, 
and asked for an explanation. Fresnel submitted the question - 
mathematical analysis, and demonstrated, that whatever < ge 
was produced by the motion of the prism in the relative velocity 
of light, the wave-length in the prism, and the refraction, = 
compensated by the physiological aberration when the ™ 
