074 SIR DAVID BREWSTER ON BINOCULAR VISION. 



whose apparent distance is d, a the angular magnitude of the remoter line, whose 

 apparent distance is D, and V, v the visual magnitudes of the two lines, then 



V: = Ax?:xD. 



Now, let the two lines MO, N P, be the two sides of a quadrilateral figure 

 seen obliquely by an eye at E, then, if the apparent distances of M O, N P, are 



such, that 



A x d -^- a x D, then V -^ v, 



and the lines M N, P, will converge to a vanishing point beyond N P. But if 



A x d = a x D, then V = , 

 and the line M N, OP, will appear to be parallel. And if 



A x d ^L a x D, then V ^. v, 



and the lines M N, OP, will converge to a vanishing point between M and the 

 observer. 



These results may be considered as laying the foundation of a new art, to 

 which we may give the name of VISUAL PERSPECTIVE, in contradistinction to 

 GEOMETRICAL PERSPECTIVE. This art furnishes us with an immediate explana- 

 tion of a great variety of optical illusions which have never yet been explained : 

 and there is reason to believe that some of its principles were known to ancient 

 architects, and even employed in modifying the nature and position of the lines 

 and forms which enter into the construction of their finest edifices. 



ST LEONARD'S COLLEGE, ST ANDBEWS, 

 April 10. 1844. 



APPENDIX. 



