THE POLAEISCOPE. 
153 
does z u). Also the light coming from z strikes P Q at the same 
angle as that at which it was before reflected by ab, namely, 
at 56|°. 
The brass ring supporting the pillars gh, Ki enables the plate 
PQ to turn bodily round the vertical axis zu. The rim bs is 
graduated to determine the angle between lm and Gi. 
A bright object placed at z will be seen by an eye at v in the 
direction vu, and Avill, as is well known, appear equally bright, 
as the mirror is turned on its axis, in all positions of the mirror. 
If, however, the object is removed to a position in the line yx, 
so that the light coming from it must be reflected by A B before 
it can fall on the mirror z, and thence to the eye at v, it will be 
found that the brightness of the image seen in p q by the eye at 
V will depend on the position of the mirror p Q. When p q, A b 
are parallel, the image of the object at x is seen bright. As 
PQ is turned, the image gets darker, until PQ and ab are, as in 
the figure, at right angles to each other. In this position the 
image disappears entirely, and P q looks perfectly black. On 
turning p Q further the image reappears, and again attains its 
maximum brightness when pq has revolved through half the 
circle. On completing the revolution of p q the same phenomena 
are repeated, the image disappearing when three-quarters of 
the circle are completed. The only difference between the light 
coming from z direct and that coming from x is that it has been 
reflected in the second case by the glass plate ab. We see that 
this reflection has given to the light peculiar properties. It is 
no longer capable of complete reflection by pq, except in two 
fixed positions of pq, and is totally incapable of being reflected 
when the latter mirror is placed in a position half way between 
those it occupied when the whole of the light was reflected. It 
is from these properties connected with direction that light pos- 
sessing them is termed polarized light. 
Various means of producing polarized light will be described. 
This of reflection at the surface of a transparent substance is 
the simplest. To completely polarize the light, the tangent of 
the angle of incidence must be equal to the refractive index of 
the medium. Light consists, there is no doubt, of the undu- 
latory motion of the particles of an ether, which is present 
wherever light passes. And further, the motion of each particle 
of light is perpendicular to the direction in which the wave is 
travelling, as is the case in the vibrations of the stretched string 
producing a musical note. 
An idea of the nature of polarized light may be gained, 
perhaps, from fig. 8. Suppose ah c to represent a line of 
particles at rest. Suppose a stream of light to pass in the 
direction ah ,, d. The particle a first begins to move, then 6, 
and so on. If a, h, c, &c. describe the same path over and over 
