152 THE SEVEN FOLLIES OF SCIENCE 



only be after considerable experience that we shall learn to 

 disregard the apparent impression that there are two balls. 



The cause of this illusion is readily found. In the ordi- 

 nary position of the fingers the same ball cannot touch at 

 the same time the exterior sides of two adjoining fingers. 

 When the two fingers are crossed, the conditions are ex- 

 ceptionally changed, but the instinctive interpretation 

 remains the same, unless a frequent repetition of the exper- 

 iment has overcome the effect of our first education on this 

 point. The experiment, in fact has to be repeated a great 

 number of times to make the illusion become less and less 

 appreciable. 



But of all the senses, that of sight is the most liable to 

 error and illusion, as the following simple illustrations will 

 show. 



In Fig. 23 a black spot has been placed on a white 



Fig. 23. Fig. 24. 



ground, and in Fig. 24 a white spot is placed on a black 

 ground ; which is the larger, the black spot or the white 

 one ? To every eye the white spot will appear to be the 

 largest, but as a matter of fact they are both the same size. 

 This curious effect is attributed by Helmholtz to what is 

 called irradiation. The eye may also be greatly deceived 

 even in regard to the length of lines placed side by side. 



