OPTICS. SI 



they would have met at H and G, if the mirror had 

 not been in the way, and will form the aforesaid 

 images, on an oiled paper stretched horizontally in 

 the direction IK; on which paper the outlines of 

 the images may be easily drawn with a black lead 

 pencil ; and then copied on a clean sheet, and co- 

 loured, as the objects themselves are by nature. 

 In this machine it is usual to place a plain glass, 

 unpolished, in the horizontal situation I K, which 

 glass receives the images of the outward objects j 

 and their outline may be traced upon it by a black 

 lead pencil. 



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 distinctly painted on the horizontal 

 glass at I K. 



The forming a horizontal image, as I K, of an 

 upright 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 perpendicular 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 

 pointy where the ray A bf falls upon it, that ray 

 will be reflected in an equal angle from the other 

 side of the perpendicular, in the line fh I. And, if 

 a perpendicular be drawn from the point g, where 

 the ray Ac g falls upon the mirror, that ray will 

 be reflected in an equal angle from the other side 

 of the perpendicular, in the line g i I. So that all 



