354 WELLS'S NATURAL PHILOSOPHY. 



„ , Double vision may be produced by pressincr 



How may don- . . .' x^ o 



bie vision be slightly from the side upon the ball of either 



produced? "-" , . . i • i 



eye while viewing an object ; the pressure of 

 the finger prevents the ball of one eye from following the 

 motion of the other, and the axis of vision in each eye 

 being rendered different, we see two images. 



Strabismus, or squinting, is caused by the inability of one eye to follow th« 

 motioiia of the other, and persons so affected always see double ; practice^ 

 however, gives them power of attending to the sensation of only one eye at a 

 time. 



It is from this inability of the eye to fix its optical axis that drunkards see 

 double. 



„ . 711. We judge of the distance and size of 



judge of the an object by the relative direction of lines 



distance and •' •' 



Bize of an ob- drawn from the object to the eye, and by the 

 angle which the intersection of these lines 

 makes with the eye. This angle is called the angle of 

 vision. 



Fig. 287. 





The student will bear in mind that an angle is simply the 



angie^of vision? inclination of two lines without any regard to their length. 



Thus, in Fig. 287, the Unes drawn from A and B, C and D, 



which may be supposed to represent rays of light, meet at the eye, and form 



an angle at the point of intersection. This angle is the angle of vision. 



If A B, Fig. 287, represent a man on a distant mountain, or on a church 

 steeple, and C D a crow close by, the angle formed by the inclination of tha 

 lines proceeding from the two objects will be equal, or the line A B, whicli a 

 the height of the man, will subtend the same angle as the line C D, which is 

 the height of the crow ; and therefore the man appears at such a distance no 

 larger than a crow. 



jT . , The nearer an object is to the eye, the greater must be the 



an^le of vision inclination of the lines drawn from its extremities to intersect. 



distance? ^^ ^°*^ ^^^''^ ^^ angle at the eye, and consequently the greater 



will be its angle of vision. On the contrary, the more remote 



