20 SECTIONAL ADDRESSES 
moves, it passes through states which have been termed ‘fits of easy 
transmission and easy reflexion’ by Newton, these states recurring 
periodically. 
The form of the undulating theory which was adopted is due to 
Huygens. On this theory light consists of undulations propagated 
through an elastic medium which fills all space ; it is assumed that the 
elasticity of this medium is different in different material bodies and 
different from its elasticity in free space, and that therefore the velocity 
of propagation of light in a material medium is different from its 
velocity of propagation in free space. It is a consequence of either theory 
that when all the media are isotropic Zy¢ along the path of a ray from 
one point to another point is stationary, and this relation is sufficient to 
give the results which are classed under the term of Geometrical Optics. 
The modification necessary in this result to make it applicable to the 
case of crystalline media was effected by Laplace, who made use of the 
corpuscular theory of light in his investigation and assumed that the 
velocity of the light particles in a crystalline medium depended on 
the direction. The same result was also derived from the undulatory 
theory. 
At the end of the eighteenth century the corpuscular theory of light 
was the theory which was accepted generally ; one of the main arguments 
against an undulatory theory was its failure to explain the formation of 
shadows. Early last century the principle of interference was put 
forward by Young to account for the formation of shadows on the un- 
dulatory theory, and somewhat later, though independently, Fresnel 
arrived at the same result. In 1816 Arago and Fresnel showed that 
light polarised in perpendicular planes did not interfere. It is not 
improbable that Fresnel had inferred already that the direction of the 
disturbance which constituted light was transverse to the direction of 
propagation, and that these experiments confirmed it, but he makes no 
reference to the principle of transversality in his writings for a con- 
siderable time. The earliest explicit reference to the principle I have 
been able to find is contained in a letter from Young to Arago written in 
January 1817. Young had visited Arago after the experiments had been 
carried out in 1816 and discussed them with him, and he appears to have 
been the only one who saw the importance of Fresnel’s inference and who 
agreed with it. In his essay on diffraction (1818) Fresnel does not refer 
to the principle ; he uses Huygens’ principle and the principle of inter- 
ference to obtain his results, principles which are independent of the 
direction of the disturbance. After the publication of his essay on 
diffraction, Fresnel applied his law of transversality to the phenomena 
of polarisation, the propagation of light in crystalline media and other 
problems. He obtained and verified by observation relations between 
the intensities of the incident, transmitted, and reflected light, when light 
is incident on a surface which separates two isotropic transparent media, 
and these relations have ever since been regarded as conditions which 
any adequate theory of light must satisfy. This is also true of the results 
he obtained for the propagation of light in crystalline media. Fresnel’s 
