258 THE EYE. 



essentially the same with the one above described, but the 

 paper on which the image is finally received, when all is 

 ready and the picture is to be taken, is made sensitive by 

 being covered with a chemical substance which is affected 

 by the light. 



There is an immense number of optical instruments, 

 greatly varied in their construction and in the purposes 

 which they serve, which, however, all depend upon the op- 

 eration of the simple principles of reflection and refraction 

 which have been explained. To enter into a description, 

 or even an enumeration of them, would be foreign to the 

 purpose of this work, which is simply to unfold and explain 

 the grand fundamental principles that are exemplified in 

 the action of light, as it exhibits itself to us in the phenom- 

 ena of nature around us. 



There is one principle which is, however, only in a sec- 

 ondary sense a property of light, which I must explain be- 

 fore closing this chapter, and that on account of the great 

 interest which John and Flippy took in it, and in making a 

 certain class of toys illustrative of it. 



The principle is called the Persistence of Vision. The 

 phrase denotes a certain property of the retina of the eye, 

 or of the nervous connection between the retina and the 

 mind, or rather, perhaps, a property of light in relation to 

 these, by which the impression made upon the mind does 

 not instantly cease when the image is made to vanish. 

 Thus a succession of very rapid flashes always appear to 

 us like a continued light, as the impression left by one does 

 not fade before another comes to renew it. 



A great many ingenious toys are constructed on this 

 principle. The kind which principally struck Flippy's fan- 

 cy chiefly, I suppose, because he could manufacture them 

 himself consisted in making two different pictures on the 

 two sides of a card, and then, by attaching strings at the 



