424 



PHYSIOLOGY 



CHAP. 



affected by the increasing height of the figure, and seems longer 

 thau it is ; line c c' appears shorter than d d', because it is 

 affected by the decreasing width of the figure. 



In Fig. 214 the three bands, a, b and c, are really the same 

 height ; but b appears higher than the others, and a lower. This 

 is because the parallel oblique lines of b are longer than the 

 vertical lines of a, and as this greater length is taken into account 

 in estimating the height of b, it appears taller. In c this illusion 

 is diminished, because the oblique lines are so close together that 

 they are no longer seen clearly apart, but look like a uniformly 



Fi.:. -214. Optical illusions. (Botti.) 



grey bundle, in which the length of the individual lines is no 

 longer prominent ; consequently the influence of irradiation 

 comes in. 



In Fig. 215 a number of horizontal lines of equal length are 

 so arranged that they form, as a whole, the shape of a pipe ; this 

 produces the illusion that the horizontal lines on the left side 

 become shorter below, and that those on the right are still 

 shorter. This deception arises because the distance between the 



FIG. 215. Modification of optical illusion in last figure. (Botti.) 



extreme ends of the pipe makes a greater impression when the 

 figure is taken as a whole than the length of the individual lines. 



In Fig. 216 three oblique lines are drawn through a series of 

 equidistant parallel straight lines. They do not, however, appeal- 

 straight, but seem to be 'divided into a number of small segments, 

 which lie more or less vertical to the parallel lines, so as to 

 produce a kind of ladder. This illusion is allied to that of 

 Poggendorff, and is due, according to Kiesow, to exaggerated 

 effect of the shortest distance between the horizontal parallels. 



The ladder -illusion, reproduced in a of Fig. 217, can be 

 abolished, or much reduced, by introducing hieroglyphs or even 

 dots upon the line that intersects the parallels, as shown in b and c 

 of the figure. In this case, the hieroglyphs or dots attract undue 

 attention to the horizontal line. 



