82 



PEOCEEDINGS OF THE AMERICAN ACADEMY. 



from less than 1 mm. up to the full diameter of the visible iris, which 

 in the darkness may retreat even within the rim of the cornea, as 

 Du Bois-Reymond 5 has shown> The eye therefore works over' an 

 aperture range varying from /20 or more down to /2.5 or/2. Inci- 

 dentally the iris, acting as a stop behind the strongly refracting cornea, 

 produces a certain amount of typical "pincushion distortion " which is 

 evident in some optical illusions. 



10 



20 30 



Meter-candles 



40 



50 



Figure 2. 



Data on the actual relation between intensity of incident light and 

 pupillary aperture are scarce and imperfect. So much depends on the 

 state of adaptation of the eye, individual sensitiveness, and probably 

 also upon the intrinsic brightness of the source, that reliable values of 

 the relation are difficult to obtain. From a reduction of Lambert's 

 data, however, I have plotted the curve of Figure 2, giving as abscissae 

 the illumination in meter-candles and as ordinates the area of the 

 pupil in square millimeters. The striking fact is at once in evidence 

 that this curve, like those of Figure 1, is rapidly becoming asymptotic in 

 the neighborhood of 10 meter-candles. In other words, the contraction 

 and expansion of the iris is less to protect the eye at high intensities 



8 Centralbl. f. prakt. Augenheilkunde, 1888. 



