448 ELPLMENTARY PHYSIOLOOV 



upon its face, be lighted sideways by a candle, we at once 

 know the head to be raised (or a cameo) by the disposition 

 of the light and shade ; and if an intaglio, or medal on 

 which the head is hollowed out, be lighted in the same 

 way, its nature is as readily judged by the eye. 



But now, if either of the objects thus lighted be viewed 

 with a convex lens, which inverts its position, the light 

 and dark sides will be reversed. With the reversal the 

 judgment of the mind will change, so that the cameo will 

 be regarded as an intaglio, and the intaglio as a cameo ; 

 for the light still comes from where it did, but the cameo 

 appears to have the shadows of an intaglio, and vice versa. 

 So completely, however, is this interpretation of the facts 

 a matter of judgment, that if a pin be stuck beside the 

 medal so as to throw a shadow, the pin and its shadow, 

 being revei'sed by the lens, will suggest that the direction 

 of the light is also reversed, and the medals will seem to 

 be what they really are. 



9. The Judgment of Changes of Torm..— Whenever 

 an external object is ivatched rapidly changing its form, a 

 continuous series of different pictures of the object is im- 

 pressed upon the same spot of the. retina. 



Conversely, if a continuous series of different pictures of 

 one object is impressed upon one part of the retina, the mind 

 judges that they are due to a single external object, under- 

 going changes of form. 



This is the principle of the curious toy called the thau- 

 matrope, or " zootrope," or " wheel of life," by the help 

 of which, on looking through a hole, one sees images of 

 jugglers throwing up and catching balls, or boys playing 

 at leapfrog over one another's backs. This is managed 

 by painting at intervals, on a disc of card, figures and 

 jugglers in the attitudes of throwing, waiting to catch, and 

 catching ; or boys " giving a back," leaping, and coming 

 into position after leaping. The disc is then made to 

 rotate before an opening, so that each image shall be 

 presented for an instant, and follow its predecessor before 



