NATURAL HISTORY. 41 



And ia the silence of his calm abode, 



In nature s works he worshipped nature s God.&quot; 



MATILDA HOUSTON. 



of the eye, which admit of great objects being successively presented 

 to it with a rapidity of which we are unconscious. 



115. It is easy to show that if the eye were without motion, steadily fixed in the 

 socket, the vision would be quickly lost ; that objects of the greatest brilliancy 

 would be obscurely seen, or disappear. For example, let us fix the eye on one 

 point a thing somewhat difficult to do, owing to the very disposition in the eye to 

 be constantly moving ; but suppose that by repeated attempts we have at length 

 acquired the power of directing the eye steadily ou an object, when we have done 

 so, we shall find that the whole scene becomes more and more obscure, and finally 

 vanishes. Let us fix the eye on the corner of the frame of the principal picture in 

 the room ; at first everything around the room will be distinct ; in a very little 

 time the impression will become weaker, objects will appear dim, and then the eye 

 will have an almost uncontrollable desire to wander ; if this be resisted, the im 

 pressions of the figures in the picture will first fade ; for a time we shall see the 

 gilded frame alone ; but this also will become faint. When we have thus ascer 

 tained the fact, if we change the direction of the eye but ever so little, the whole 

 scene will at once again be perfect to us. 



116. Why is incessant motion of the eye essential to the continued 

 exercise of the organ ? 



Because when the eye is fixed upon a point, the lights, shades, 

 and colours of objects continuing to strike upon the same part of the 

 retina, the nerve is exhausted ; but when the eye shifts there is a 

 new exercise of the nerve ; the part of the retina that was opposed 

 to the lights is now opposed to the shades, and what was opposed to 

 different colours is now opposed to other colours, and the variation 

 in the exciting cause produces a renewed sensation. 



117. Why do we know that the ideas we obtain of the size, shape, 

 and distance of objects depend mainly upon the education of the 

 sight? 



Because optical illusions, however nearly they resemble realities 

 when they first meet the eye, are satisfactorily proved to have no 

 existence, by the attentive and correct use of that very organ 

 which gave rise to the error. 



118. Dr. Cheselden, by a surgical operation, procured sight to a very intelligent 

 person who was born blind, and he observed the manner in which this sense was 

 developed in the young man. &quot; When he saw the light for the first time, he knew 

 BO little how to judge of distances, that Li believed the objects which he s%w 



