THATJMATKOPES. 



of the retina when in a state of repose than when its entire surface 

 is excited by surrounding lights. Thus it is found that while the 

 varying duration of the impression of the illuminated object in a 

 dark room was one-third of a second, its duration in a lighted 

 room was only one-sixth of a second. 



67. Innumerable optical toys and pyrotechnic apparatus owe- 

 their effect to this continuance of the impression upon the retina 

 when the object has changed its position. 



Amusing toys, called thaumatropes, phenakisticopes, phantas- 

 kopes, &c., are explained upon this principle. A moving object, 

 which assumes a succession of different positions in performing any 

 action, is represented in the successive divisions of the circumfer- 

 ence of a circle, as in fig. 8, in the successive positions it assumes. 

 These pictures, by causing the disk to revolve, are brought in 

 rapid succession before an aperture, through which the eye is 

 directed, so that the pictures representing the successive attitudes 

 are brought one after another before the eye at intervals ; the* 

 impression of one remaining until the impression of the next is 

 produced. In this manner the eye never ceases to see the figure, 

 but sees it in such a succession of attitudes as it would assume if 

 it revolved. The effect is, that the figure actually appears to 

 pirouette before the eye. The effects of Catherine-wheels and 

 rockets are explained in the same manner. 



68. The direction in which any part of an object is seen is that 

 of the line drawn from such point through the optical centre of 

 the eye. This line being carried back to the retina determines 

 the place on the retina where the image of such point is found. 

 If the optical centre of the eye were not at the centre of the eye- 

 ball, the direction of this line would be changed with every move- 

 ment of the eye-ball in its socket ; every such movement would 

 cause the optical centre to revolve round the centre of the eye- 

 ball, and consequently would cause the line drawn from the 

 optical centre to the object to change its direction. The effect of 

 this would be that every movement of the eye-ball would cause 

 an apparent movement of all visible objects. Now, since there is 

 no apparent motion of this kind, and since the apparent position 

 of external objects remains the same, however the eye may be moved 

 in its socket, it follows that its optical centre must be at the centre 

 of the eye-ball. 



69. Since lines drawn from the various points of a visible 

 object through the centre of the eye remain unchanged, however 

 the eye-ball may move in its socket, and since the corresponding 

 points of the image placed upon these lines must also remain 

 unchanged, it follows that the position of the image formed on 

 the eye remains fixed, even though the eye-ball revolve in the 



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