THE PERCEPTION OF SIGHT. 307 



causes of mistake as to the position of the objects we 

 see defective accommodation when looking through one 

 or two small openings, improper convergence when 

 looking with one eye only, irregular position of the 

 eye-ball from external pressure or from paralysis of its 

 muscles. Moreover, illusions may come in from certain 

 elements of sensation not being accurately distinguished ; 

 as, for instance, the degree of convergence of the two 

 eyes, of which it is difficult to form an accurate judgment 

 when the muscles which produce it become fatigued. 



The simple rule for all illusions of sight is this : we 

 always believe that we see such objects as would, under 

 conditions of normal vision, produce the retinal image 

 of which we are actually conscious. If these images are 

 such as could not be produced by any normal kind of 

 observation, we judge of them according to their nearest 

 resemblance; and in forming this judgment, we more 

 easily neglect the parts of sensation which are imperfectly 

 than those which are perfectly apprehended. When more 

 than one interpretation is possible, we usually waver 

 involuntarily between them ; but it is possible to end 

 this uncertainty by bringing the idea of any of the 

 possible interpretations we choose as vividly as possible 

 before the mind by a conscious effort of the will. 



These illusions obviously depend upon mental processes 

 which may be described as false inductions. But there 

 are, no doubt, judgments which do not depend upon 

 our consciously thinking over former observations of the 

 same kind, and examining whether they justify the 

 conclusion which we form. I have, therefore, named 

 these 'unconscious judgments;' and this term, though 

 accepted by other supporters of the empirical theory, 

 has excited much opposition, because, according to 

 generally-accepted psychological doctrines, & judgment, 



