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28 LIGHT AND WATER
from A to C, and no farther. The lines of vision in
this diagram take what we may call their xormal
directions, z.¢., those which they would follow after
reflexion in a horizontal surface, as that of still water.
Now if the mirror be slightly tipped up, as in Fig.
11b,' the lines of vision will all be shifted upwards
from their normal directions ; the reflexion of the top
of the picture appears no longer in its normal position
at C, but has retired to A, whilst at B and C we see
the wall above, and at D the ceiling.’ (In order to
see the bottom of the picture reflected in the tilted
mirror, it would be necessary to move the table for¬
ward and place the mirror at K.) So, in the case of
a succession of waves coming towards or receding
from us, it is evident that, as we look upon their near
sides, we shall frequently see nothing but the sky
reflected, whereas, had the water been smooth, we
should in the same direction have seen the reflexions
of objects on the opposite shore.
Fig. 12 is intended to illustrate roughly the way
in which this effect is produced. Suppose that a man
" Here represented in each position as tilted through an angle
of 5°.
* The effect isthe morenoticeable owing to the fact that if a mirror
on which a ray falls is rotated (on an axis at right angles to the plane
of incidence) through any angle, the reflected ray moves through
twice that angle. For if the mirror be turned through any angle 8,
the normal to it is turned through the same angle. Hence the angle
between the ray and the normal is increased or diminished by 6),
and therefore that between the incident and reflected rays (which
is double of this) is altered by twice 0.
In the same way, if one of the mirrors in Fig. 11a 1s tilted through
a given angle, the line of vision undergoes deflexion through twice
that angle.