772 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



results of this persistence is the combination into a single image, of two or 

 more objects presented to the eye in successive movements ; but these must 

 be of a kind which can be united, otherwise a confused picture is produced. 

 Thus in a little toy, called the Thaumatrope, which was introduced some 

 years ago, the two objects were painted on the opposite sides of a card, a 

 bird, for instance, on one, and a cage on the other; and when the card was 

 made (by twisting a pair of strings) to revolve about one of its diameters, in 

 such a manner as to be alternately presenting the two sides to the eye at 

 minute intervals, the two pictures were blended, the bird being seen in the 

 cage. A far more curious illusion, however, was that first brought into 

 notice by Professor Faraday; who showed that, if two toothed wheels, placed 



FIG. 275. 



m 



\\* 



^ ' 



$ 



13 



s :/ 

 ' 



r 



S / 



NK' 



I 



v > y 



one behind the other, be made to revolve with equal velocity, a stationary 

 spectrum will be seen ; whilst if one be made to revolve more rapidly than 

 the other, or the number of teeth be different, the spectrum also will revolve. 

 The same takes place when a single wheel is made to revolve before a mir- 

 ror, the wheel and its image answering the purpose of the two wheels in the 

 former case. On this principle, a number of very ingenious toys have been 

 constructed : in some of these, the same figure or object is seen in a variety 

 of positions; and the successive impressions, passing rapidly before the eye, 

 give rise by their combination to the idea that the object is itself moving 

 through these positions. 1 It is interesting to remark, moreover, that when 



1 A very beautiful "philosophical toy" was shown to the Author some years since, 

 by its inventor, Mr. Roberts, the celebrated machinist of Manchester: consisting in 

 an apparatus by which it was made possible to read words printed on a card, although 

 the r;m1 itself 'was made to revolve on its axis even 40,000 times in a minute. The 

 principle of its construction was simply this, that the eye caught a succession of 

 jlimpMs <.f (lie card, through a narrow slit before which a disk with 11 single corre- 

 sponding perforation was made to revolve; the rats of movement of this disk being so 

 adjusted to that of the card, that whenever the eye caught .sight of the latter, it was 

 jijomo//^/-/'/// iii the same position, so that, by the succession of transient impressions 

 thus made upon the retina, the words printed on the card could be distinctly read. 



