*37

Of Optics.

number of times the picture is longer or broader than the objectjand the length multiplied by the breadth, stiews how much the wholesurface is magnified.

Before we enter upon the defcription of telescopes, it will be proper Tehscepu.to ffiew how the rays of light are affected by paffing thron gh concaveglaffes, and also by falling upon concave mirrours.

When parallel rays, as abcdefgh, pafs directly through a glafs Pkte XVIII.AB, which is equally concave on both sides, they will di verge aster Fi S-paffing through the glafs, as if they had come from a radiant point C,in the center of the glafs’s concavity; which point is called the ne-gative or virtual focus of the glafs. Thus the ray a, aster paffingthrough the glafs AB, will go on in the direction k l, as if it had pro-ceeded from the point C, and no glafs been in the way. The rayb will go on in the direction tnn ; the ray c in the direction op, &c.—

The ray C, that falls directly upon the middle of the glafs, suffers norefraction in paffing through it; but goes on in the fame rectilinealdirection, as if no glafs had been in its way.

If the glafs had been concave only on one side, and the other sidequite plane, the rays would have diverged, aster paffing through it,as if they had come from a radiant point at double the distance of Cfrom the glafs; that is, as if the radiant had been at the distance of awhole diameter of the glafs’s concavity.

If rays come more converging to such a glafs, than parallel rays di-verge aster paffing through it, they will continue to converge asterpaffing through it; but will not meet so soon as if no glafs had beenin the way; and will incline towards the fame side to which theywould have diverged, if they had come parallel to the glafs. Thus therays f and h, going in a converging state towards the edge of the glafs at

B, and converging more in their way to it than the parallel rays di vergeaster paffing through it, they will go on converging aster they pafsthrough it, though in a lefs degree than they did before, and will meetat I: but if no glafs had been in their way, they would have met at i.

When parallel rays, as df a, Cmb, e Ic, fall upon a concave mir- Fig. z.rour A B (which is not transparent, but has only the surface A b B ofa ciear polisti) they will be restected back from that mirrour, and meetin a point m, at half the distance of the surface of the mirrour from

C, the center of its concavity : for they will be restected at as greatan angle from a perpendicular to the surface of the mirrour, as they feliupon it, with regard to that perpendicular; but on the other side there-of. Thus, let C be the center of concavity of the mirrour Ab B, andlct the parallel rays df a, Cmb , and elc, fall upon it at the points a, b,

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