346 



PRACTICAL PHYSIOLOGY. 



[lxxiii. 



but the after-images are inclined against the inclination of the vertical 

 images. 



Suppose we look at a rectangular red cross (p) under the same circumstances 

 (fig. 263), on turning the eyes, i.e., the visual line, to any vertical or hori- 

 zontal line passing through p, the after-image is a rectangular cross, but it 

 appeal's oblique, and its angles are neither horizontal nor vertical when the 

 eyes look obliquely, i.e., when the point of vision diverges considerably from 

 the above-named lines. The apparently displaced crosses are shown in a, 6, 

 c, d. 



These oblique after-images were formerly regarded as showing that the 

 eyeball rotated on its antero-posterior axis, i.e., " wheel movements.''^ This is 

 not the case, the movements are only apparent. If they were real the after- 

 images ought to move in the same direction with both vertical and horizontal 

 strips, but they do not. 



4. Irradiation. — By irradiation is meant the fact that, under 

 certain circumstances, objects appear larger than they should be 

 according to their absolute size and distance from the eye, larger 



^^^^^ than other objects of greater 



/^ ^^\ ^^^^^^^^ or less brightness of the 



same size and at the same 

 distance. 



{a.) Cut out two circles 

 ,^^^^^^^^^ as in fig. 264, or two squares 

 \ / ^^^^^^^^r of exactly the same size, of 



^-^ -^ ^^^^^^^ white and of black paper. 



Fig. 264.— Irradiation. Place the white patch on a 



black, and the black on a 

 white sheet of paper. Hold them some distance from the eye, and, 

 especially if they be not distinctly focussed, the white circle will 

 appear larger than the black one. 



•) 



FlQ. 265. 



FlO. 266. 



{h.) Divide a square into four, as shown in fig. 265, two of the 

 smaller squares being white and two black. Hold the figure at 

 some distance from you. The two white squares appear larger, and 



