96 



POPULAR FALLACIES. 



number, yet the number distinctly seen by the naked eye at any one time, un- 

 aided by the telescope, is not great. Any one can satisfy himself of this 

 by examining any good map of the stars ; yet, when we look at the firmament 

 on a clear night, these objects appear to be inconceivably numerous. This 

 illusion is dispelled by examining the heavens through the most ordinary tele- 

 scope, or even by looking through a long tube, which will limit the view at any 

 one moment to a small portion of the firmament. On the entire sphere of the 

 heavens there are not above twenty stars of the first magnitude, and it is seldom 

 that as many as six or eight of these can be seen at once. The number of 

 stars of the second magnitude does not exceed fifty, and of these twenty are 

 seldom seen at any one time. The stars of the third magnitude may amount 

 to about two hundred, half of which only can be at the same time above the 

 horizon. The smaller stars are much more numerous, but they are discernable 

 with difficulty, and do not produce upon the mind the impression of multitude 

 that we conceive. 



I have explained, on another occasion, that the membrane of the eye, which 

 is affected by light, retains the impression it has received for about the tenth 

 of a second after the cause which produced the impression has been removed. 

 When a lighted stick is whirled in a circle, the circle will appear to be one 

 continuous line of light, because the eye retains the impression which the light J 

 produces upon it at any point in the circle until the stick returns to that point. 

 The light is, therefore, visible at the same time at every point of the circle. 



Ingenious optical toys are constructed, the effects of which are explicable on 

 these principles. The same object is painted on the several divisions of cir- 

 cumference of a circle in a succession of different attitudes, and while the eye 

 is directed to the highest point of the circle, through an opening made for that 

 purpose, the circle is made to revolve, and the object passes before the eye in 

 a succession of different attitudes. If the velocity with which the circle turns 

 be such that the eye shall retain the impression of the object in one attitude 

 until its picture in another attitude comes into view, it will have all the effect 

 of a moving object. Waltzing figures and other similar devices are painted on 

 circular cards and mounted, so as to give these effects. 



If the eye is supplied with no external means of knowing the distance of a 

 visible object, it estimates that distance by its apparent magnitude, and if there 

 be any means of causing the magnitude of the same objects to undergo a grad- 

 ual change, the impression on the spectator is as if the object advanced to or 

 receded from him. It is upon this principle that the exhibitions of phantasma- 

 goria are made. The image of an object is formed on some surface prepared 

 to receive it, the apartment being elsewhere in complete darkness, so that the 

 observer has no means of knowing where the image is formed. The magic 

 lantern has a power, by advancing it gradually toward the surface, to diminish 

 the size of the image indefinitely, and by drawing it from the surface to aug- 

 ment it. The spectators, therefore, see the images gradually increase and di- 

 minish, and imagine it gradually to approach to and recede from them. 



