SENSE ORGANS. 



147 



vision. Such a supposed surface of single vision with the 

 point of sight fixed is called the horopter. Whether there be 

 such a surface at all, and if there be, what is its form, 



B' 



m 



\n 



CL' 



n 



a 



Fig. 92. — The horopteric circle of Muller : R and L, two eyes ; n «', point 

 of crossing of ray lines — nodal point ; A, point of sight ; B, some other 

 point in the horopteric circle A nri ; aa', central spots ; aa', bb', ret- 

 inal images of A and B. 



are very complex and difficult questions which can not 

 be discussed here.* 



The Relation of the Chiasm to Corresponding 

 Points. — The union of the optic nerves to form a 



* They are fully discussed in author's book Sight. 



