LET. XIX. AND ABERRATION OF LIGHT. Ziz

into an empty bason, and retire to such a dis-tance, that the edge of the bason shall just hideit from your sight; then, keeping yourselfsteady, let another person fill the vessel gentlywith water ; and as the water rises towards thetop, the object will become more and morevisible, till at length the whole of it will bedistinctly seen, appearing as if it had been raisedabove the bottom of the bason.

This proves that the rays of light are refracted,or bent downwards, in their passage out of thewater into the air; and as they now come tothe eye in a more oblique direction, the objectmust necessarily appear to be elevated, and in adifferent situation from that in which it wasreally placed. The fame thing may also beshown thus: place the bason in such a mannerthat the fun may shine obliquely on it, and ob-serve where the shadow of the rim falls uponthe bottom ; then fill it with water, and theshadow will not extend so far as it did when thevessel was empty; which shows that the rayshave changed their direction, by passing out ofone medium into another of a different density.

The less obliquely the rays fall, the less theywill be refracted; and if they fall perpendicu-larly, they will not be refracted at. all. For, inthe last experiment, the higher the fun rises, theless will be the difference between the placeswhere the edge of the shadow falls, in the empty

and