I 2 Ö

Of Optfcs.

upon the fame part of the bottom, whether the vessel be empty or füll ;which ffiews, that the rays of light are not refracted when they fallperpendicularly on the surface of any medium.

The rays of light are as much refracted by paffing out of water intoair, as by paffing out of air into water. Thus, if a ray of light flowsfrom the point e, under water, in the direction e B ; when it comes tothe surface of the water at B, it will not go on thence in the rectilinealcourse B d, but will be refracted into the line B a. Therefore,

To an eye at e looking through a plane glass in the bottom of theempty vessel, the point a cannot be seen, because the side Bc of thevesiel interpoles; and the point d will just be seen over the edge of thevessel at B. But if the vessel be filled with water, the point a will beseen from e ; and will appear as at ..d, elevated in the direction of theray eB*.

The days are The time of fun-riliilg or fetting, fupposing its rays fussered no re-j™ a ^ 1 ° n s er fraction, is easily found by calculation. But obfervation proves thattion o/the^" f un r ^ es f° oner > and fets later every day than the calculated time ;sun’s rays. the reafon of which is plain, from what was faid immediately above.

For, though the sun’s rays do not come part of the way to us throughwater, yet they do through the air or atmofphere, which being agrosser medium than the free fpace between the fun and the top of theatmofphere. the rays, by entering obliquely into the atmofphere, arethe re refracted, and thence bent down to the earth. And although thereare many places of the earth to which the fun is vertical at noon, andconfequently his rays can suffer no refraction at that time, because theycome perpendicularly through the atmofphere; yet there is no place towhich the sun’s rays do not fall obliquely on the top of the atmofphere,st his ri/ing and fetting; and confequently, no ciear day in which thefun will not be visible before he rifes in the horizon, and aster be fets init: and the longer or Ihorter, as the atmofphere is more or lefs repleteFig. 3. with vapours. For, let ABC be part of the earth’s surface, DEF theatmofphere that covers it, and EBGH the sensible horizon of an ob-server at B. As every point of the sun’s surface sends out rays of lightin ali manner of directions, forne of his rays will constantly.fall upon,and enlighten, forne half of our atmofphere; and therefore, when the'fun is at I, below the horizon H y thofe rays which go on in the free

* Hence a piece of money lying at e , in the bottom of an empty vessel, cannot beseen by an eye at a, because the edge of the vessel intervenes; but let the vessel be filled'with water, and the ray e a being then refracted at JS, will strike the eye at <7, and io»ender the money visible, which will appear as if it were raifed up to/ in the line*Bf,

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