1921.] 
Farr.—Relativity. 
233 
when light is reflected from a polished surface it makes equal angles with 
the surface before and after reflection was well enough known ; but though 
it was, of course, known that light would pass through transparent sub¬ 
stances such as water or glass, yet the law obeyed was not known until 
Descartes discovered it. It is to him that we owe the now thoroughly well 
known relation Sin (f> = ^ sin c j>'. The discovery of this relation immedi¬ 
ately gave rise to a discussion—in which a little later on Newton was a 
commanding figure—a discussion as to the meaning of the bending of light, 
which this relation means. The rather small scientific world of that day 
became divided into two opposing camps. Both parties admitted the 
necessity of a medium, but one side, led rather by Newton, contended that 
light was a form of corpuscular movement, and that the bending was due 
to the corpuscles travelling more rapidly through the glass or other dense 
medium than they did through free space; while the other school held that 
light was a form of wave-motion, and that the bending was due to the slower 
velocity of the waves in such a dense body as, say, glass as compared with 
their velocity through free space. Owing largely to the great weight of 
Newton’s authority, this controversy was carried on long after his death. 
The objections which he put forward were perfectly valid, in that those 
holding the wave theory in his day regarded their waves of light as akin to 
the waves of sound, and it was not until a century later that Thomas Young 
and Fresnel advanced the theory which ever since their day has met with 
universal acceptance, and which regards light as more or less like the waves 
of the sea—transverse waves—and not, as are the waves of sound, longi¬ 
tudinal waves of compression and rarefaction. 
Thus were the foundation-stones of our belief in the existence of an 
ether laid some three hundred years ago. Crude and very often erroneous 
as these early ideas were, they have formed the basis of discussion and 
have acted as the guide for further experiment, until in the middle years 
of the last century our knowledge of its properties might be considered as 
fairly complete. The velocity of propagation of light-waves through the ether 
had been measured by four different and quite independent methods, and 
the results had been found to agree surprisingly, though its magnitude was 
stupendous, amounting as it does to no less than 186,000 miles a second, 
and each of the four separate and independent methods had given it about 
this value. The length of the waves for different-coloured lights had been 
measured, and it was found that for a given colour the length of the wave 
was the same under whatever varied circumstances it might be measured, 
thus showing that what we call “ colour ” was merely a question of the 
length of the waves which our eyes or instruments were receiving. The 
wave theory of light, which in its essence involves the existence of an ether— 
a something, that is, for the waves to exist in—the theory of Fresnel and 
Young—had received practically universal assent by the middle of last 
century. And well it might, for by its aid the colours of the oil film on the 
pavement, the beauties of the soap-bubble, the varied and complicated 
phenomena of polarized light, had all received complete and satisfactory 
explanation. And, more than this, phenomena had been predicted—for 
example, external and internal conical refraction—which were subsequently 
found to exist. If any theory or explanation of a series of diverse and 
varied phenomena could ever be accepted, surely the undulatory theory of 
light could claim that distinction. And this theory involves the existence 
of an ether through which light has been shown, by four different methods, 
to travel at a speed of 186,000 miles a second. 
