750 EVENING DISCOURSES. 
EVENING DISCOURSES. 
FRIDAY, SEPTEMBER 6. 
Radiations Old and New. By Professor W. H. Braaa, F.R.S. 
Tue remarkable properties of the new rays which have been examined with 
such eagerness in recent years throw a curious and interesting light on the older 
attempts to find a satisfactory theory of radiation. Newton and Huygens, 
Young and Fresnel discussed various theories rejecting or adopting, and each 
has given his reasons for his final choice. It is instructive at this present time 
to examine those reasons, and to consider the influences which prompted them 
to make their great discoveries. 
The advances which Newton and Huygens made at the close of the seven- 
teenth century were based in part on the discovery that the propagation of 
light was not instantaneous, as Des Cartes had conceived. The latter 
philosopher supposed light to be a continuous pressure merely tending to move- 
ment, a pressure which was exerted across a plenum reaching from the luminous 
object to the eye. Huygens was unable to conceive how the Cartesian theory 
could explain the mutual penetration of rays of light. ‘It is impossible,’ he 
says, ‘so to understand what I have been saying about two persons mutually 
seeing one another’s eyes, or how two torches can illuminate each other.’ * 
Thus Newton and Huygens introduced the idea of the motion of some sort 
of matter as a fundamental point in their theories. ‘They did so in different 
ways; and the distinction grew to be a cleavage between two schools of thought. 
It was not a very deep distinction at first: it would have been easy to have 
stepped from one side to the other of the dividing line. Only in later times 
did the corpuscular and wave theories stand immovable in hostile antagonism. 
It is not at all impossible that modern research will once more draw the two 
theories together. 
The difference may be put in this way. ‘The former imagined the light 
corpuscle to move from the source of light to the recipient: as we should now 
say, the carrier of the energy remained unchanged throughout the process. 
On the other hand, Huygens imagined the disturbance to be passed on from 
particle to particle of the ether; that is to say, the energy was carried by relays 
of particles. It must be remembered that he thought of the ether as a collection 
of particles resembling but smaller than the particles of luminous bodies. The 
latter particles he supposed to float in a subtle medium which agitated them and 
made them strike against the particles of the ether, which thus became the seat 
of a spreading pulse. To a certain extent the difference between the ideas of 
Newton and Huygens might be likened to the difference between the despatch 
of a message by a special runner and the propagation of a rumour. This would, 
however, scarcely go far enough, for Huygens was obliged to introduce the 
important notion of ‘springiness’ in order to develop his theory. We may see 
how this necessity arose. 
There were two properties of light which seemed to Huygens to be of first- 
rate importance, and his first object was to shape this theory to explain them. The 
cne was the extreme speed of light which Romer had recently deduced from 
observations of Jupiter’s satellites; the other was the way in which rays of 
light traversed each other without hindrance. Accordingly, he rejected the 
idea of the transport of matter—the corpuscular theory—since he could not 
* “Treatise on Light,’ by Christiaan Huygens, translated by Silvanus P. 
Thompson, p. 22. 
