OF OPTICS. 269 



and then copied on a clean sheet, and coloured by art LECT 

 as the objects themselves are by nature. In this 

 machine, it is ujual to place a plane glass, unpolished, 

 in the horizontal situation / K, which glass receives the 

 images of the outward objects ; and their outlines may be 

 traced upon it by a black-lead pencil. 



N. B. The tube in which the convex glass C D is 

 fixed, must be made to draw out, or push in, so as to 

 adjust the distance of that glass from the plane mirror, 

 in proportion to the distance of the outward objects ; 

 which the operator does, until he sees their images dis- 

 tinctly painted on the horizontal glass at I K. 



The forming a horizontal image, as / K, of an up- 

 right object A B, depends upon the angles of incidence 

 of the rays upon the plane mirror E F, being equal to 

 their angles of reflection from it. For, if a perpendicu- 

 lar be supposed to be drawn to the surface of the plane 

 mirror at e, where the ray A a C e falls upon it, that ray 

 will be reflected upwards in an equal angle with the other 

 side of the perpendicular, in the line e d I. Again, if a 

 perpendicular be drawn to the mirror from the point f, 

 where the ray Abf falls upon it, that ray will be re- 

 flected in an equal angle from the other side of the per- 

 pendicular, in the line/ h I. And if a perpendicular be 

 drawn from the point g, where the ray A c g falls upon 

 the mirror, that ray will be reflected in an equal angle 

 from the other side of the perpendicular, in the line gil. 

 So that all the rays of the pencil a be, flowing from the 

 upper extremity of the object A B, and passing through 

 the convex glass C D, to the plane mirror E P, will be 

 reflected from the mirror, and meet at /, where they will 

 form the extremity / of the image / K, similar to the 

 extremity A of the object A B. The like is to be un- 

 derstood of the pencil q r s, flowing from the lower ex- 

 tremity of the object A B, and meeting at K (after re- 

 flection from the plane mirror (the rays form the extre- 

 mity K of the image, similar to the extremity B of the 



