PERSPECTIVE. 377 



Visual rays are the rays of light which come 

 from the different parts of the objects to our eyes. 



Point of sight is the spectator's eye. This has 

 been erroneously confounded with the centre of 

 the picture, as will be seen afterwards. 



What is called an original line, is any line in 

 Nature, or in the objects themselves, which are to 

 be drawn in perspective. 



An original plane is any surface or plane of the 

 objects to be represented. 



If we suppose a line to proceed from the eye, 

 parallel to any line in the objects we are viewing, 

 and to continue till it arrive at the picture or 

 perspective plane, the point where it would touch 

 the plane is called the vanishing point of the line. 



All lines that are in Nature parallel to each 

 other have the same vanishing points. The reason 

 of this will be easily seen, if the reader considers 

 the method of finding the vanishing point of an 

 original line just mentioned; for the same line 

 which would find the vanishing point of one, will 

 do for them all, and form only one point. 



If we could suppose a plane or surface to pro- 

 ceed from the eye of the spectator, in a direction 

 parallel to any side of an object which we view 

 through the plane of glass, and if it continue / till 

 it arrive at the glass, the line which it would form 

 by contact with the glass, is called a vanishing line ; 

 and it is the vanishing line of the side of the 

 object which the supposed plane was parallel to. 



In every picture or perspective plane, there is a 

 point, where a line drawn from the eye perpen- 

 dicular to the picture, would touch it ; this point 

 is called the centre of the picture, and is the same 

 which is often called in old books on perspective, 

 the point of sight. But this is a wrong term for it ; 



