SCIENCE OF COMMON THINGS. 267 



Vision in the light and in darkness. Cats how see in the dark. 



Because the pupil (which contracted in the bright 

 room) does not dilate instantaneously ; and the con- 

 tracted pupil is not able to collect rays enough from the 

 darker road or street to enable us to see objects before us. 



IT'S? How does light cause the pupil of the eye to contract ? 



The pupil of the eye is a round hole in the midst of 

 a movable muscular curtain or screen, called the iris. 

 "When too much light falls on the nervous retina at the 

 back of the eye, it irritates it ; and this irritation is 

 conveyed to the muscular rings composing the curtain 

 by small nervous fibres, causing them to contract. 



17*38 Why do we see better when we get used to the dark f 



Because the pupil dilates again, and allows more rays 

 to pass through its aperture.; in consequence of which 

 we see more distinctly. 



17*39 If we look at the sun for a few moments, why do all oilier things 

 appear dark f 



Because the nerve of the eye, by looking at the sun, 

 is so affected by the intensity of the light that it requires 

 a few moments to recover its former sensibility. 



17*30 Why can we see the proper colors of every object again after a 

 few minutes f 



Because the eye again recovers its sensibility, and 

 accommodates itself to the light around. 



17*31 Why can tigers, cats, and owls see in the dark f 



Because they have the power of enlarging the pupil 

 of their eyes so as to collect the scatterea rays of light ; 

 in consequence of which they can see distinctly when it 

 is not light enough for us to see anything at all. 



17*33 Why is it that when we press slightly upon the ball of either eye, 

 while viewing an object, we see double? \ 



Because 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 different, we see 

 two images. 



17*33 Do persons who squint see double 1 



They do / but practice gives them p^ower of attending 

 to the sensation of only one eye at a time. 



