Stranica 73 [73]
REFLEXIONS IN RIPPLED WATER 41
greased side down, over a cloth, with the result that,
on lifting it, its surface is smeared in parallel straight
lines. The glass is then placed, greased side up¬
wards, so that the light from a candle is reflected from
it to the eye at an oblique angle; and, first, with the
lines of grease across the direction ofvision. A long,
well-defined streak
will be seen to run
vertically through
the image of the
candle flame thus
(Fig.17a). Now let
the glass be turned
round. The streak
will gradually be¬
come inclined, but
very slowly, so that
when the glass has
been turned through
half aright angle, the
streak will only have
reached the position
shown in Fig. 17b. Fig. 17a. Reflexion of a candle flame
in greased glass (lines of grease parallel
to AB).
When, however, the
glass hasbeen turned
through a whole right angle, so that the lines of
grease lie along the direction of vision, there will
remain only a very short horizontal streak of light
through the image, as in Fig. 17c.
We have here a rude imitation of the effect pro¬
duced by reflexion from very regular ripples. In each
figure the lines of grease run parallel to AB and CD.