1894.] 



Defining Power of Composite Eyes. 



89 



to the surface at the ends of a measured chord, choosing the chord 

 so that the surface outside it should have fairly uniform curvature. 

 The length of the chord was usually about three-quarters, or a little 

 more, of that of the eye. 



Taking the length of the chord as Z, and r as radius of the sphere 

 which best represents the surface of the eye, we have for the angle 

 of view 6, 



sin^e = Z/2r, 



