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150

/i 'E:.

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Fig.

Of Optics.

line produCed to E, beyond the glass. And a ray BD, flowing fromhis foot, and falling obliquely on the glass at D, will be reflected asobliquely on the other side of the perpendicular ab D, in the directionDA ; and the foot of his image will appear at F, in the direction of thereflected ray A D, produced to F, where it is cut by the right line'BGFy drawn parallel to the right YmeACE. Jnst the fame as if theglass were taken away, and a real man stood at F y equal in size to theman standing at B: for to his eye at A, the eye of the other man at Ewould be feen in the direction of the line ACE j and the foot of the manat F would be feen by the eye A, in the direction of the line ^ O F.

If the glass be brought nearer the man A B , as fuppose to cb, hewill fee his image as at CDG: for the reflected ray CA (being perpen-dicular to the glass) will shew the eye of the image as at C; and theincident ray B b, being reflected in the line b A, will Ihew the foot ofhis image as at G; the angle of reflection a b A being always equal tothe angle of incidence Bba: and so of all the intermediate rays fromA to B. Hence, if the man A B advances towards the glass CD, hisimage will approach towards it; and if he recedes from the glass, hisimage will also recede from it

Having already fhewn, that the rays of light are refracted when theypass obliquely through different mediums, we come now to prove thatforne rays are more refrangible than others; and that, as they are diffe-rently refracted, they excite in our minds the ideas of different colours.This will account for the colours feen about the edges of the images ofthofe objects which are viewed through forne telefcopes.

Lct the fun fhine into a dark room through a small hole, as at e e ,in a window-stiutter; and place a triangulär prisin B C, in the beam ofrays A, in such a manner, that the beam may fall obliquely on one ofThe/?//». the sides ab C of the prisin. The rays will suffer different refractionsby pasiing through the prifm, so that instead of going ali out of it onthe side dcC, in one direction, they will go on from it in the differentdirections reprefented by the lines f g, h, i, k, /, m, n ; and falling uponthe opposite side of the room, or on a white paper placed as at pq , toreceive them, they will paint upon it a series of most beautiful lively co-The colours e/dours (not to be equalled by art) in this order, viz. thofe rays which areleast refracted by the prifm, and will therefore go on between the linesn and m, will be of a very bright intense red at n, degenerating fromthence gradually into an orange colour, as they are nearer the line m:the next will be of a fine orange colour at m, and from thence dege-nerate into a yellow colour towards l: at / they will be of a fine yellow,which will incline towards a green, more and more, as they are nearer. and

the light.