Of Optics. 129

V plane’surface. But the; rays CG and CI will be so refracted, as tqmeet again at the point F. Therefore, all the rays which flow fromthe point C, so as to go through the glass, will meet again at F ; andif they go farther onward, as to L, tbey cross at F, and go forwardon the opposite sides of the middle ray CDEF , to what they werein approachiiig it in the directions HF and KF. i

When parallel rays, as ABC, fall directly upon a plano-convex Fi g . s.glass DE, and pass through it, they will be so refracted, as to unite The proper-in a point f behind it; and this point is called the principal focus ; ties °fdiffe-the distance of which, from the middle of the glass, is called the rent en ei 'Jbcal dißance ; which 1 is equal to twice the radius of the sphere of theglass’s convexity. And,

When parallel rays, as ABC, fall directly upon a glass DE, which Fig. 9. ris equally convex on both sides, and pass through it; they will be sorefracted, as to meet in a point or principal focus f, whofe distanceis equal to the. radius or semidiameter of the sphere of the glase’sconvexity. But if a glass be more convex on one side than on th^other, the rule for finding the focal distance is this; as the sum of thesemidiameters of both convexities is to the semidiameter of either, sois double the semidiameter of the other to the distance of the focusf.

Or, divide the double product of the radii by their sum, and thequotient will be the distance sought. }

Since all thofe rays of the fun which pass through a convex glast arecollected together in its focus, the force of all their heat is collectedinto that part; and is in proportion to the common heat of the fun,as the area of the glase is to the area of the focus. Hence we fee thereafon why a convex glase caufes the fun’s rays to burn after pasiingthrough it.

All these rays crose the middle ray in the focus f, ''and then divergefrom it, to the contrary sides, in the fame manner FfG, as they con-verged in the space D/E in coming to it.

If another glase FG, of the same convexity as D E, be placed inthe rays at the same distance from the focus, it will refract them so,as that after going out of it, they will be all parallel, as abc ; and <

go on in the same manner as they came to the first glase DE, throughthe space ABC-, but on the contrary sides of the middle ray Bfb:for the ray ADf will go on from/' in the direction fGa, and theray CEf in the direction fFc ; and so of the rest.

The rays diverge from any radiant point, as from a principal focus :therefore, if a candle be placed at f, in the focus of the convexglase FG, the diverging rays in the space FfG will be so refracte ( d

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