EVENING DISCOURSES. 751. 
imagine how matter could move with so great a speed, nor could he conceive that 
material rays might be able to traverse each other. He arranged his ether cor- 
puscles in a row between the source and the receiver, and compared the move- 
ment of light with the passage of a disturbance along a row of glass spheres. 
Jn this way he planned to give any desired velocity to the wave; for, as he says, 
there is nothing to hinder us from supposing the ‘particles of the ether to be of 
a substance as nearly approaching to perfect hardness and possessing a 
springiness as prompt as we choose. And further, the mutual traverse could be 
illustrated by directing two other glass spheres against the ends of the row— 
with equal but opposite velocities and in the line of the row—whereupon each 
sphere was reflected with its original speed. ‘The idea of springiness thus intro- 
duced was sufficient in addition to explain the fact that the velocity of light 
was independent of its intensity. 
It is to be observed that the ideas of hardness and impenetrability are 
‘important factors in Huygens’ explanations. 
The difference between the ideas of Newton and Huygens develops itself 
in another very important direction. The theory of the former implied the 
transference of energy from place to place without spreading by the way. 
That of the latter led naturally to spreading; and indeed too much so, as it 
seemed to Newton, who rejected it mainly on that ground. Huygens was really 
unable to meet Newton’s obiection, tor his famous construction of the wave- 
front as un envelope of secondary waves is rather empirical than deductive. 
The construction was a true one, nevertheless, and it led him to some of his most 
brilliant discoveries, such as his determination of the wave-front in Iceland spar. 
Newton held it as one of his main objects to explain the varieties of colour. 
He had discovered that colour constituted an essential quality or distinction of 
light which could not be altered by reflection or refraction, and he had to 
account in some way for a variation which did not depend on the speed in 
space. He investigated the colours of thin sheets of transparent materials, 
such as soap-films, and, as is well known, he supposed that the light corpuscle 
went through regular successive stages of easy retlection and easy transmission, 
and ‘that the length of those stages varied with the colour. In this way he 
introduced the idea of periodicity and the direct connection of colour with 
periodicity which are such important factors of the wave theory. A soap-film 
which was of such a thickness that red light was in a fit of easy reflection at 
the second face, was not related in the same way to green light, and hence 
the colours of the films. With colours Huygens did not concern himself. More- 
cver, he only considered the single pulse. The wave-train, the interference 
principle, and the transverse vibration were introduced by Young and Fresnel. 
Let us now consider some of the properties of the rays from radioactive 
substances. 
In the case of the a particle we have a movement of matter which belies all 
that Huygens conceived to be essential to its nature. The a particle is an atom, 
as material as anything else, and yet it travels with a speed comparable to that 
of light. One of the two main reasons which Huygens gave for rejecting the 
corpuscular theory falls at once. Moreover, as long as it moves with this 
excessive speed it is able to penetrate other atoms with ease; and so the second 
reason is greatly shaken. 
These remarkable effects are worth a closer examination. The usual method 
of investigating the behaviour of any of the new rays is based on their electrical 
effects. As an a particle passes through a gas it leaves behind a trail along 
which are to be found certain atoms which have lost electrons, and other atoms 
ty which the separated electrons have attached themselves. By collecting these 
debris from the various places where they are formed we can map fut the 
whole path of the particle and learn its history in various circumstances. We 
have been able to show that the path is an almost perfectly straight line, broken 
only by rare deflections, and those chiefly at the end of the track when the speed 
is less. Such a path imolies that the a particle has passed through many 
thousands of atoms. For it could not have pushed them all out of the way, 
being too light to do so; and it could not have gone round the various atoms, 
since it has no sense to recover a line once lost. The methods by which the 
properties of the a particles were first discovered are, however, somewhat 
