Sir David Brewster on Hemiopsy, or Half- Vision, 503 



31 r l 



the case if, for instance, ab' . dd -^% 

 and ad! . bd also lie in a line 

 through P. 



We have the points (a, b, d) 

 in a line, and the points (b 1 , d, d!) 

 in a line; the points a, d, b 1 , d 

 are also called A, B', B, A' re- 

 spectively ; ad', bb' meet in C, bd, 

 dd' in C; hence, considering the 

 hexagon ad'db'bd, the lines 



ad', b'b meet in C , 

 d'd, be' „ C , 



db', ca' „ AA'.BB'; 



and hence these three points lie in a line ; or, what is the same 

 thing, the lines AA', BB', and CC meet in a point, that is, the 

 triangles ABC, A'B'C are in perspective; the corresponding 

 sides are 



AB, A'B', that is, ab', dd meeting in aV . dd, 



BC, B'C „ b'b, d'd „ P , 



CA, C'A' „ ad', bd „ ad' .bd; 



and these three points lie in a line; that is, the points ab' . dd 

 and ad' . bd lie in a line through P. Hence the line through 

 ab' . dd and db . d!c and the line through ad' . bd and dd . cb' are 

 one and the same line ; that is, 



the points ab 1 . dd, a'b . d'e, ad' . bd, dd . b'c lie in a line through P. 



This proves the existence of one of the lines through P; and that 

 of the other two lines follows from the symmetry of the figure ; 

 it thus appears that the twelve points lie four together on three 

 lines through P. 



Cambridge, April 11, 1865. 



LXIX. On Hemiopsy, or Half-vision. 

 By Sir David Brewster, K.H., F.R.S.* 



THE affection of Half-vision, or Half-blindness as it has been 

 called, was first distinctly described by Dr. Wollaston, in 

 a paper " On Semidecussation of the Optic Nerves/' published 

 in the Philosophical Transactions for 1824. " It is now more 



* From the Transactions of the Royal Society of Edinburgh, vol. xxiv. 

 part 1. Communicated by the Author. 



