THE EYE. 



the object shall be formed on the retina. This picture must be 

 sufficiently illuminated to affect the sense, and at the same time 

 not be so intensely illuminated as to overpower the organ. 



Thus it is possible to conceive a picture on the retina so 

 extremely faint as to be insufficient to produce sensation, or, on 

 the other hand, so intensely brilliant as to dazzle the eye, to 

 destroy the distinctness of sense, and to produce pain. 



When we direct the eye to the sun, near the meridian, in an 

 unclouded sky, we have no distinct perception of his disk, because 

 the splendour is so great as to overpower the sense of vision, just 

 as sounds are sometimes so intense as to be deafening. 



That it is the intense splendour alone which prevents a distinct 

 perception of the solar disk in this case is rendered manifest by 

 the fact that if a portion of the solar rays be intercepted by a 

 coloured glass, or by a thin cloud, a distinct image of the sun wi 1 

 be seen. 



When we direct the eye to the firmament on a clear night, 

 there are innumerable stars which transmit light to the eye, and 

 which therefore must produce some image on the retina, but of 

 which we are altogether insensible, owing to the faintness of the 

 illumination. That the light, however, does enter the eye and 

 arrive at the retina is proved by the fact that if a telescope be 

 directed to the stars in question, so as to collect a greater quantity 

 of their light upon the retina, they will become visible. 





