VIII 



OCULAE MOVEMENTS 



425 



These optico-geometrical illusions are closely related to the 

 simpler errors of judgment, known as irradiation errors. Even 

 when accommodation of 



the dioptric system is ^ EE 



perfect, it is impossible 

 entirely to avoid vision 



by diffusion-circles. A ^^ ^ 



luminous point, such as 

 a fixed star, is perceived 

 by us as a small, round, 

 shining surface. Gener- 

 ally speaking, luminous 

 surfaces on a dark back- 

 ground appear larger 

 than they really are, for 

 instance a white square or 

 circle on a black ground 



seems larger than a black ^ ^ _ == 



square or circle of the ^ -^ _ 



same size on a white 



FIG 216 Poggendorff-Hering's optical illusion. 

 (Modified by Botti.) 





ground. The bright edge 

 of a crescent moon seems 

 to form part of a circle of wider radius than its shaded portion. 

 If the lower half of a candle flame is hidden by a sheet of paper, 



the luminous part seems to 

 spread over its edge, so that 

 the latter appears notched. 



All these effects, writes 

 Helmholtz, can be referred 

 to the fact that the margins 

 of illuminated surfaces seem 

 to project into the visual 

 field and to invade the ad- 

 jacent dark surfaces. Ob- 

 viously, this irradiation, the 

 blurring of contours, and the 

 consequent optical illusions 

 increase with the size of the 

 diffusion -circles which fall: 

 on the retina owing to any 

 defect of accommodation. 

 Ovio succeeded in photo- 

 graphing the principal alterations in the images caused by 

 diffusion-circles. 



Helmholtz attempted to explain these optico-geometrical 

 illusions partially by irradiation phenomena. Einthoven formu- 

 lated a theory of the gradual spread of the retinal excitations 



FIG. 217. Modifications of Poggendorf-Hering'; 

 optical illusion. (Botti.) 



