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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 upwards, 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 become 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 produced by reflexion from very regular ripples. In each figure the lines of grease run parallel to AB and CD.
