149

Of Optics.

The image of any object that is placed before a plane mirrour, ap- The commonpears as big to the eye as the object itself$ and is erect, distinct, and koking-giafi.feemingly as far behind the mirrour, as the object is before it; andthat part of the mirrour, which reflects the image of the object tothe eye (the eye being suppofed equally distant from the glass withthe object) is just half as long and half as broad as the object itself. LetAB be an object placed before the reflecting surface gh i of the plane Fig. z.mirrour CD ; and let the eye be at o. Let Ab be a ray of lightstowing from the top A of the object, and falling upon the mirrour ath ; and h m be a perpendicular to the fur face of the mirrour at h: theray A h will be reflected from the mirrour to the eye at o, making anangle m h o equal to the angle Ahm: then will the top of the imageE appear to the eye in the direction of the reflected ray o h producedto E, where the right line ApE, from the top of the object; ciitsthe right line o h E, at E. Let Bi be a ray of light proceeding fromthe foot of the object at B to the mirrour at i ; and ni a perpendicu-lar to the mirrour from the point i, where the ray B i falis upon it: thisray will be reflected in the line / o, making an angle n i o, equal to theangle B i n, with that perpendicular, and entering the eye at o: thenwill the foot F of the image appear in the direction of the reflected rayo i, produced to F, where the right line B F cuts the reflected ray pro-duc ed to F. All the other rays that flow from the intermediate pointsof the object AB, and fall upon the mirrour between h and i, will bereflected to the eye at o; and all the intermediate points of the image E Fwill appear to the eye in the direction of thefe reflected rays produced.

But all the rays that flow from the object, and fall upon the mirrourabove b, will be reflected back above the eye at o-, and ali the raysthat flow from the object, and fall upon the mirrour below /, will bereflected back below the eye at o : so that none of the rays that fallabove b, or below i, can be reflected to the eye at o ; and the distancebetween h and i is equal to half the length of the object A B.

Hence it appears, that if a man fees his whole image in a plane Amanwiiiseelooking- glass, the part of the glass that reflects his image must be just 3

half as long and half as broad as himfelf, let him stand at any distance ing-giasi, thatfrom it whatever; and that his image must appear just as far behind ,s but half histhe glass as he is before it. Thus, the man A B viewing himfelf in the e ‘ s £ 'plane mirrour CD, which is just half as long as himfelf, fees his whole Fig. 4.image as at E F, behind the glass, exactly equal to his own size. For,a ray AC, proceeding from his eye at A, and falling perpendicuiarly 1upon the surface of the glass at C, is reflected back to his eye in thefame line C A-, and the eye of his image will appear at E, in the fame

line