POWER AND STRUCTURE OF THE EYE. 95 



LETTER II. 



The Eye the most important of our organs Popular description of it 

 The eye is the most fertile source of mental illusions Disappear- 

 ance of objects when their images 'fall upon the base of the optic 

 nerve Disappearance of objects when seen obliquely Deceptions 

 arising from viewing 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 accidental 

 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 

 Disappearance of halves of objects or of one of two persons In~ 

 sensibility of the eye to particular colours Remarkable optical 

 illusion described. 



OF all the organs by which we acquire a knowledge of 

 external nature the eye is the most remarkable and the 

 most important. By our other senses the information we 

 obtain is comparatively limited. The touch and the taste 

 extend no further 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 bound- 

 less range of observation. It takes cognizance not only 

 of other worlds belonging to the solar system, but of other 

 systems of worlds infinitely removed into the immensity 

 of space ; and when aided by the telescope, the invention 

 of human wisdom, it is able to discover the forms, the 

 phenomena, and the movements of bodies whose distance 



