280 



EXPERIMENTAL PHYSIOLOGY. 



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. 130, or two squares of 



exactly the same 

 size, of white and 

 of black paper. Place 

 the white patch on a 

 black, and the black 

 on a white sheet of 

 paper. Hold them 

 some distance from 

 the eye, and espe- 

 Fig. 130. Irradiation. cially if they be not 



distinctly focussed, the white circle will appear larger than 



the black one. 



(b.) Divide a square into four as shown in Fig. 131, two 

 of the smaller squares being white and two black. Hold the 



Fig. 131. 



Fig. 132. 



figure at some distance from you. The two white squares 

 appear larger, and they appear to run into each other and 

 to be joined together by a white bridge. 



(c.) Look at Fig. 132, placed at such a distance that the 

 accommodation is imperfect. The white stripe which is of 

 equal breadth throughout appears wedge-shape, being wider 

 below between the broad black patches, and narrow above. 

 To me also the narrow black patches appear to be broader 

 above and narrower below. 



