PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 531 
the direction of the vibrations in polarized light. So long as the suspended particles 
are large compared with the waves of light, reflexion takes place as it would from a 
portion of the surface of a large solid immersed in the fluid, and no conclusion can 
be drawn either way. But if the diameters of the particles be small compared with 
the length of a wave of light, it seems plain that the vibrations in a reflected ray 
cannot be perpendicular to the vibrations in the incident ray. Let us suppose for 
the present, that in the case of the beams actually observed, the suspended particles 
were small compared with the length of a wave of light. Observation showed that 
the reflected ray was polarized. Now all the appearances presented by a plane- 
polarized ray are symmetrical with respect to the plane of polarization. Hence we 
have two directions to choose between for the direction of the vibrations in the 
reflected ray, namely, that of the incident ray, and a direction perpendicular to both 
the incident and the reflected rays. The former would be necessarily perpendicular to 
the directions of vibration in the incident ray, and therefore we are obliged to choose 
the latter, and consequently to suppose that the vibrations of plane-polarized light 
are perpendicular to the plane of polarization, since experiment shows that the plane 
of polarization of the reflected ray is the plane of reflexion. According to this 
theory, if we resolve the vibrations in the incident ray horizontally and vertically, 
the resolved parts will correspond to the two rays, polarized respectively in and per- 
pendicularly to the plane of reflexion, into which the incident ray may be conceived 
to be divided, and of these the former alone is capable of furnishing a reflected ray, 
that is of course a ray reflected vertically upwards. And in fact observation shows, 
that, in order to quench the dispersed beam, it is sufficient, instead of analysing the 
reflected light, to polarize the incident light in a plane perpendicular to the plane of 
reflexion. 
Now in the case of several of the beams actually observed, it is probable that many 
of the particles were really small compared with the length of a wave of light. At 
any rate they can hardly fail to have been small enough to produce a tendency in the 
polarization towards what it would become in the limit. But no tendency what- 
soever was observed towards polarization in a plane perpendicular to the plane of 
reflexion. On the contrary, there did appear to be a tendency towards a more com- 
plete polarization in the plane of reflexion. 
M. Babinet has been led by the same reasoning to an opposite conclusion 
respecting the direction of the vibrations in polarized light, resting on an experiment 
of M. Arago’s, in which it appeared that when light was incident perpendicularly on 
the surface of white paper, and the reflected or rather scattered light was viewed in 
a direction almost grazing the surface, it was found to be partially polarized in the 
plane of the sheet of paper*. But the actions which take place when light is inci- 
dent on a broad irregular surface, like that of paper, bounding too a body which is 
so translucent that a great part of the light must enter it and come out again, appear 
* Comptes Rendus, tom. xxix. p. 514. 
