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SECTION A—MATHEMATICAL AND PHYSICAL SCIENCES. 
THEORIES OF LIGHT 
ADDRESS BY 
PROF. H. M. MACDONALD, O.B.E., LL.D., F.R.S., 
PRESIDENT OF THE SECTION. 
EarLy speculations as to how impressions were produced on the senses 
ascribed the sensations associated with the senses of taste and smell to 
the emanation of small particles of the substances involved, and ascribed 
the sensations associated with the sense of sound to undulations or pulses 
inthe air. 'The sensations associated with the sense of sight were assumed 
by some philosophers to be produced in a manner similar to those belong- 
ing to the senses of taste and smell, while by others they were assumed to 
be produced in a manner similar to those of sound. In the first case 
they were assumed to be produced by emanations from the body seen, 
in the second case by undulations due to the body. Among the Greeks 
Empedocles was an exponent of the first view, while Aristotle supported 
the second view. It should be noted that different views were held by 
those who supported an emanation theory as to the nature of the emana- 
tion. Some held that the emanation consisted of small particles of 
matter, while others held that the emanation was something different from 
matter. 
In the fifteenth and sixteenth centuries, when attention was being 
directed again to the study of natural phenomena, the two types of theory 
were revived. The form of the emanation theory which was adopted 
ultimately is that due to Newton, usually referred to as the corpuscular 
theory of light. In this theory light is regarded as consisting of very 
small particles of matter emitted by luminous bodies with the same 
velocity, the velocity of light. These light particles are supposed to be 
repelled or attracted by the molecules of material bodies according to 
some law depending on the distance between them, It is further assumed 
that the law is such that the force can change from an attraction to a 
repulsion or from a repulsion to an attraction, that these forces are 
insensible at sensible distances, that the motion of a light particle 
satisfies the ordinary laws of dynamics, and that, as the light particle 
