Chap. 3-] Reflexion from plane Mirrors. 191 



hind the glafs as the object is before it. To under- 

 ftand perfectly the reafon of this, it will be neceflary 

 to advert to the fubject of vifion as explained for- 

 merly in a note. It will be remembered, that by the 

 fpherical form of the eye, and particularly by means 

 of the cryilalline humour which is placed in the middle 

 of it, the rays of light are converged, and thofe from 

 the extreme points of the object crofs each other, fo as 

 to form an inverted image on that part of the optic 

 nerve which is called the retina. The apparent mag- 

 nitude of objects will conlequently depend upon the 

 fize of the inverted image, or, in other words, upon 

 the angle which the rays of light form, by entering 

 the eye from the extremities of any object. 



As therefore the angle of reflexion is always equal 

 to the angle of incidence, it will be evident on the 

 infpection of Plate VI. fig. i, that the converging 

 rays K m, L , proceeding from the extremities of 

 the object K L, and falling on the mirror a b, are re- 

 flected to the eye at e with the fame degree of conver- 

 gence, and confequently will caufe the image k I to 

 be feen under an angle equal to that under which 

 the object itfelf would nave b^en feen from the point 

 / without the interpolation of the mirror. The image 

 appears alib at a diftance behind the mirror equal to 

 that at which the object (lands before it. For it muft 

 be remembered, that objects are rendered vifible to 

 our eyes not by a fingle ray proceeding from every 

 point of an object, but that in fact pencils or aggre- 

 gates of divergent rays proceed from every point of 

 all viiible objects, which rays are again by the mecha- 

 nifm of the eye converged to a point on all thofe parts 

 of the retina whjre the image is depicted. The point 

 from which the rays diverge is called the focus of di- 

 vergent rays 3 and the point behind a reflecting furface 

 from which they appear to diverge is called the -vir- 

 tual 



