NUMBER AND DISTANCE. 



eernible with difficulty, and do not produce upon the mind the 

 impression of multitude that we conceive. 



8. It has been ascertained that the membrane of the eye, 

 which is affected by light, retains the impression it has re- 

 ceived 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 con- 

 tinuous line of light, because the eye retains the impression 

 which the light 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 the circumference of a circle in a succes- 

 sion 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. 



9. 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 object to undergo a gradual 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 phantasmagoria 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 placed. The magic 

 lantern has a power, by advancing it gradually toward the 

 .nil-lace, to diminish the size of the image indefinitely, and by 

 drawing it from the surface to augment it. The spectators, 

 therefore, seeing the images gradually increase and diminish 

 imagine they gradually approach to and recede from them. 



10. Although the eye, by its direct as well as its indirect 

 indications, supplies the greatest variety of impressions, of which 

 many admit of exact numerical estimation, the touch, according 

 to popular notions, is regarded as a more sure test of reality. 

 The incredulous apostle, who refused to believe the evidence of 

 his eyes, yielded to that of his touch. This sense is, never- 



89 



