374 THE SENSORIAL FUNCTIONS. 



too extensive to be treated at len^jth 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 judgment correctly, 

 but are then acting under unusual or irregular combinations 

 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 chiss includes those illusions in which an impres- 

 sion is really made on tlic organ of sense by an external 

 cause, but in away to wiiicli we have not been accustomed. To 

 this class belong the acoustic deceptions arising from echoes, 

 and from the art of ventriloquism; the deceptive appear- 

 ances of the mirage of the desert, the looming of the horizon 

 at sea, the Fata Morgana of the coast of Calabria, the gi- 

 gantic spectre of the Brockcn in the Hartz, the suspended 

 images of concave mirrors, the visions of the phantasmago- 

 ria, the symmetrical reduplications of objects in the field of 

 the kaleidoscope, and a multitude of other results of the 

 simple combinations of the laws of optics. 



The second class comprehends those in which the cause 

 of deception is more internal, and consists in the peculiar 

 condition of the nervous surface receiving the impressions. 

 Ocular spectra of various kinds, impressions on the tongue 

 and the eye from galvanism, and those which occasion sing- 

 ing in the ears, arising generally from an excited circulation, 

 are among the many perceptions which rank under this 

 head. 



The third class of fallacies comprehends those which are 

 essentially niental in their origin, and are the consequences 

 of errors in our reasoning powers. Some of these have al- 

 ready been pointed out with regard to the perceptions of 

 vision and of hearing, the formation of which is regulated 



• In the Gulstonian Lectures, whicli I was appointed to read to the Royal 

 College of Physicians, in May, 1832, 1 took occasion to enlarge on this sub- 

 ject. A summary of these lectures was given in the London Medical Ga- 

 zette, vol. X. p. 273, 



