89 
LIGHT AND THE ETHER. 
By Professor A. D. Ross, M.A., D.Sc., F.R.A.S., F.R.S.E, 
(Bead 13th May , 1919.) 
The wave theory of light was advanced by Huyghens towards 
the close of the 17th century. It gradually superseded the emis- 
sion or corpuscular theory which had been held by Newton, but 
which was at variance with the fact that the velocity of light is 
greater in air than in denser media such as water and glass. The 
medium in which the light waves take place on HuygheiFs theory 
is termed the aether — a subtle iluid which permeates all space. 
And since the velocity of light in air is the same as that of an 
electromagnetic wave in air. it is concluded that light itself is an 
electromagnetic wave. This conclusion has been generally accepted 
as the result of Hertz’s experiments and Clerk-Maxwell 
mathematical investigations. 
The velocity of light has been determined in a variety of 
ways: — 1. From Romer’s observations of the acceleration and 
retardation of the times of occuiTen.ce of eclipses of Jupiter’s 
satellites depending upon the varying, distance of the Earth from 
Jupiter. *2. From Bradley’s investigation of the aberration of 
light — light appearing to come from a direction slightly different 
from the -true direction in consequence of the Earth ’s motion (the 
effect is strictly analogous to the phenomenon of vertically falling 
lain appearing to come obliquely from in front against a person 
moving through it). 3. From experiments by Foucault, Fizeaii, 
Forbes, and others on the time taken by light to travel over a 
measured distance not exceeding a few miles. 
In Bradley’s investigations of the aberration of light, it was 
shown that a telescope used in observing a star was always slightly 
inclined to the true direction of the star by an amount depending 
upon the ratio of the Earth’s velocity to the velocity of light. The 
theory assumed that the aether was at rest while the observing 
telescope and the contained air moved through it. As the experi- 
ment gave a result in harmony with those of other methods, this 
assumption was evidently justified. Airy, however, repeated 
Bradley's experiment with the telescope filled with water. Since 
light travels in water with only three-fourths of its velocity in air, 
the aberration should have been correspondingly greater. It was 
found, however, to be quite unaltered. Apparently, then, air 
moves freely through the aether, but water drags the aether along, 
tresnel made a mathematical investigation of this aether drift, 
and his resulting formula was afterwards verified by an ingenious 
