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Of Optics.

candles without confusion.—Dr. Niewentyt has computed, that thereflows more than 6,000,000,000,000 times as many particles of Iightfrom a candle in one fecond of time, as there are grains of fand in thewhole earth, fupposing each cubic inch of it to contain 1,000,000.

These particles, by falling directly upon our eyes, excite in ourminds the idea of light. And when they fall upon bodies, and arethereby reflected to our eyes, they excite in us the ideas of these bo-dies. And as every point of a visible body reflects the rays of light inall manner of directions, every point will be visible in every part towhich the light is reflected from it. Thus the object ABC is visible Piate XV.to an eye in any part where the rays A a, Ab, Ac, Ad, A e, Ba, Bb, Fig. 1.Bc, Bd, Be, and Ca, Cb, Cc, Cd, Ce, come. Here we have shewnthe rays as if they were only reflected from the ends A and B, andfrom the middle point C of the object; every other point being sup-posed to reflect rays in the fame manner. So that, wherever a spectator Reflectedis placed with regard to the body, every point of that part of the fur-face which is towards him will be visible, when no intervening objectstops the passage of the light.

Linee no object can be seen through the bore of a bended pipe, it isevident that the rays of light move in straight lines, whilst there is no-thing to refract or turn them out of their rectilineal courfe.

Whilst the rays of light continue in any * medium of an uniformdensity, they are straight; but when they pafs obliquely out of one me-dium into another, which is either more dense or more rare, they arerefracted tawards the denser medium: and this refraction is more or less,as the rays fall more or less obliquely on the refracting surface whichdivides the mediums.

To prove this by experlment, let the empty vessel ABCD into any Fig. 2.place where the fun fhines obliquely, and obferve the part where theIhadow of the edge B C falis on the bottom of the vessel at E ; then£11 the vessel with water, and the lhadow will reach no farther than e ;which stiews, that the ray aBE, which came straight in the open air,just over the edge of the vessel at B to its bottom at E, is refracted by Refractedfalling obliquely on the surface of the water at B ; and instead of going h g ht -on in the rectilineal direction aBE, it is bent downward in the waterfrom B to e; the whole bend being at the surface os the water: andso of ali the other rays a bc.

If a stick be laid over the vessel, and the fun’s rays be reflected froma glafs perpendicularly into the vesiel, the shadow of the stick will fall

* Any thing through which the rays of light can pafs, is called a medium; as air,water, glafs, diamond, or even a vacuum.

upon.