LETTEES ON NATUEAL MAGIC. 



LETTER II. 



The eye the most important of our organs Popular descrip- 

 tion of it The eye is the most fertile source of mental 

 illusions Disappearance of objects when their images fall 

 upon the base of the optic nerve Disappearance of ob- 

 jects when seen obliquely Deceptions arising from view- 

 ing objects in a faint light Luminous figures created by 

 pressure on the eye, either from external causes or from 

 the fulness of the blood-vessels Ocular spectra or acci- 

 dental colours Remarkable effects produced by intense 

 light Influence of the imagination in viewing these 

 spectra Remarkable illusion produced by this affection 

 of the eye Duration of impressions of light on the eye 

 Thaumatrope Improvements upon it suggested Dis- 

 appearance of halves of objects or of one of two persons 

 Insensibility of the eye to particular colours Remark- 

 able optical illusion described. 



OF all the organs by which we acquire a know- 

 ledge of external nature, the eye is the most 

 remarkable and the most important. By our 

 other senses the information we obtain is com- 

 paratively limited. The touch and the taste 

 extend no farther than the surface of our own 

 bodies. The sense of smell is exercised within 

 a very narrow sphere, and that of recognizing 

 sounds is limited to the distance at which we 

 hear the bursting of a meteor and the crash of a 

 thunderbolt. But the eye enjoys a boundless 

 range of observation. It takes cognizance not 



