G04: ADDITIONS. 



hypothesis that the transverse vibrations which constitute polarization 

 are perpendicularly transverse to the plane of polarization was con 

 firmed. 1 



But Mr. Holtzmann, 2 who, assenting to the reasoning, has made the 

 experiment in a somewhat different manner, has obtained an opposite 

 result ; so that the point may be regarded as still doubtful. 



Final Disproof of the Emission Theory. 



As I have stated in the History, we cannot properly say that there 

 ever was an Emission Theory of Light which was the rival of the 

 Undulatory Theory : for while the undulatory theory provided expla- 

 nations of new classes of phenomena as fast as they arose, and exhi- 

 bited a consilience of theories in these explanations, the hypothesis of 

 emitted particles required new machinery for every new set of facts, 

 and soon ceased to be capable even of expressing the facts. The sim- 

 ple cases of the ordinary reflexion and refraction of light were explained 

 by Newton on the supposition that the transmission of light is the 

 motion of particles : and though his explanation includes a somewhat 

 harsh assumption (that a refracting surface exercises an attractive force 

 through & fixed finite space), the authority of his great name gave it a 

 sort of permanent notoriety, and made it to be regarded as a standard 

 point of comparison between a supposed " Emission Theory " and the 

 undulation theory. And the way in which the theories were to be 

 tested in this case was obvious : in the Newtonian theory, the velocity 

 of light is increased by the refracting medium ; in the undulatory 

 theory, it is diminished. On the former hypothesis the velocity of 

 light in air and in water is as 3 to 4 ; in the latter, as 4 to 3. 



But the immense velocity of light made it appear impossible to 

 measure it, within the limits of any finite space which we can occupy 

 with refracting matter. The velocity of light is known from astrono- 

 mical phenomena ; from the eclipses of Jupiter's satellites, by which 

 it appears that light occupies 8 minutes in coming from the sun to the 

 earth ; and from the aberration of light, by which its velocity is shown 

 to be 10,000 times the velocity of the earth in its orbit. Is it, then, 

 possible to make apparent so small a difference as that between its 

 passing through a few yards of air and of water ? 



Mr. Wheatstone, in 1831, invented a machine by which this could 



: Camb. Trans., vol. ix. part i. 1849. 2 Phil. Mag., Feb. 1857 



