532 THE SENSORIAL FUNCTIONS. 



also black. The green appearance of the letters, 

 in this case, was an optical illusion, arising from 

 the tendency of the retina, which had been 

 strongly impressed with red light, to receive im- 

 pressions corresponding to the complementary 

 colour, which is green. 



A philosophical history of the illusions of the 

 senses would afford ample evidence that limits 

 have been intentionally assigned to our powers 

 of perception ; but the subject is much too ex- 

 tensive to be treated at length in the present 

 work.* I must content myself with remarking, 

 that these illusions are the direct consequences 

 of the very same laws, which, in ordinary cir- 

 cumstances, direct our judgment correctly, but 

 are then acting under unusual or irregular com- 

 binations of circumstances. These illusions may 

 be arranged under three classes, according as 

 they are dependent on causes of a physical, 

 physiological, or mental kind. 



The first class includes those illusions in 

 which an impression is really made on the 

 organ of sense by an external cause ; but in a 

 way to which we have not been accustomed. 

 To this class belong the acoustic deceptions 

 arising from echoes, and from the art of ven- 



* In the Gulstonian Lectures, which I was appointed to read 

 to the Royal College of Physicians, in May, 1832, I took occa- 

 sion to enlarge on this subject. A summary of these lectures 

 was given in the London Medical Gazette, vol. x. p. 273. 



