ILLUSIONS OF THE SENSES. 47.3 



the tendency of the retina, which had been strongly 

 impressed with red light, to receive impressions 

 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 per- 

 ception ; but the subject is much too extensive 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 circumstances, direct our judg- 

 ment correctly, but are then acting under unusual 

 or irregular combinations of circumstances. These 

 illusions may be arranged under three classes, ac- 

 cording as they are dependent on causes of a phy- 

 sical, 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 belona: 

 the acoustic deceptions arising from echoes, and 

 from the art of ventriloquism ; the deceptive ap- 

 pearances of the mirage of the desert, the looming 

 of the horizon at sea, the Fata Morgana of the 

 coast of Calabria, the gigantic spectre of the 

 Brocken in the Hartz, the suspended images of 

 concave mirrors, the visions of the phantasmagoria, 

 the symmetrical reduplications of objects in the 

 field of the kaleidoscope, and a multitude of other 



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

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

 enlarge on this subject. A summary of these lectures was given in 

 the London Medical Gazette, vol. x. p. 273. 



