23 . Two sides of a Thaumatrope disc. 



Having given a long account of the result of M. Helmholtz's labours, 

 we will pass to the consideration of another kind of experiments, or rather 

 appliances, based on the illusions of vision, and the persistence of impressions 

 on the retina. 



The Thaumatrope, to which we have already referred, is a plaything of 

 very ancient origin, based on the principle we have mentioned. It consists 

 of a cardboard disc, which we put in motion. by pulling two cords. On one 

 side of the disc a cage, a, is portrayed, on the other a bird, b (fig. 1 2 3). When 

 the little contrivance is turned round, the two designs are seen at the same 

 time, and form but one image that of a bird in its cage (fig. 1 24). It is of 

 course hardly necessary to add that the designs may be varied. 



We have already referred to M. Plateau's rotating disc (the Phenakisto- 

 scope). Through the narrow slits we perceive in succession representations 

 of different positions of a certain action. The persistence of the luminous 

 impressions on the retina gives to the eye the sensation of a continuous 

 image, which seems animated by the same movements as those portrayed 

 in the different phases (fig. 125). 



The Zootrope (fig. 126) is a perfected specimen of this apparatus. It is 

 composed of a cylinder of cardboard, turning on a central axis. The cylin- 

 der is pierced with vertical slits at regular intervals, and through which the 



THE THAUMATROPE, 121 



the eyes in the apparent direction of the lines, M. Helmholtz, after discussing 

 the matter very thoroughly, proves the strengthening of the illusion in 

 Zollner's illustration to be caused by those motions. It is not now our 

 intention to follow out the whole of this demonstration ; it will be sufficient 

 to point out to the reader a fruitful force of study, with but little known 

 results. | 



The Romans were well acquainted with the influence of oblique lines. 

 At Pompeii, fresco paintings are to be found, in which the lines are not 

 parallel, so that they satisfy the eye influenced by adjacent lines. En- 

 gravers in copper-plate have also studied the influence of etchings on the 

 parallelism of straight lines, and they calculate the effect that they will 

 produce on the engraving. In some ornamentations in which these results 

 have not been calculated, it sometimes happens that parallel lines do not 

 appear parallel because of the influence of other oblique lines, and a dis- 

 agreeable effect is produced. A similar result is to be seen at the railway 

 station at Lyons, the roof of which is covered with inlaid work in point de 

 Hongrie. The wide parallel lines of this ceiling appear to deviate, a result 

 produced by a series of oblique lines formed by the planks of wood. 



