aper on to the glass or by means of a little sealing-wax. Stick a pin into the board, the head of the pin beingat the same height above the board as the mark. Look at thefront face of the glass directly, from behind the pin, withthe eye placed at a slightly higher level than the top of thepin. The… … Fig. 58. Test the coincidence by slightly shifting the eye about.Let P be the top of the pin and let Q be the image of the pinand of the mark P. Then PQP is a straight line normal tothe front of the glass, let it cut it in A. Then since Q is a 45] REFRACTION AT PLANE SURFACES. 79 reflected image of…
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