268 SCIENCE OF COMMON THINGS. 



Cause of squinting. We see images and not objects themselves. 



17*34 What is the cause of strabismus, or squinting ? 



The inability of one eye to follow the motions of the 

 other ; this may arise from habit, imperfect power in 

 one eye, or some defect in the muscular movements. 



17*35 Why, when the eye is violently struck or pressed upon, do we seem 

 to see light ? 



Because the pressure communicated to the optic 

 nerve causes a violent and momentary sensation of 

 light. 



17*3G When we say we see an object, what do we in fact do f 



The mind is only taking cognizance of the picture or 

 impression made on the retina. 



17*37* If the mind, in seeing an object, sees in reality only a picture 

 yainted on the back of the retina, how is it enabkd to judge of magnitudes, 

 distances, etc., the picture being on a comparatively fiat surface f 



It is only by experience. " I see men as trees walk- 

 ing," said the man born blind, when restored to sight. 



17*33 Would a person whose eyes, although perfect, had been covered up 

 from infancy to maturity, be able to see ? that is, comprehend any scene or 

 prospect on which he first opened his eyes ? 



He would see the objects, but could no more under- 

 stand them than a child understands the printed page 

 on which it looks, although every word is clear and 

 distinct. 



17*39 Do we see the same lines and surfaces of an object alike wicheach 

 eye? 



"We do not. 



We may convince ourselves that we do not, by placing two candles, fo? 

 example, in such & position, that when they are looked at with the right eye, 

 one is made to cover the other ; if now we close the right eye and look at 

 them with the left, the most remote candle will be no longer screened by 

 the front one, but will be seen about an inch to the left of it. 



17*40 Why cannot we count the posts of a fence when wt are riding 

 rapidly in a railroad car ? 



Every impression, according to the intensity of its 

 effects, remains for a certain length of time on the 

 retina, and a measurable period is necessary to produce 

 the impression. The light from each post falls upon 

 the eye in such rapid succession, that the different 

 images become confused and blended, and we do not 

 obtain a distinct vision of the particular parts. 



