OPTICAL DELUSIONS. 



549 



figures pasted upon both sides. A larger disc, of 25 cm diameter, is 

 provided with twelve apertures at equal distances from one another 

 close to the edge, and each l cm wide. Both discs are perforated in 

 the middle by a hole 6 mm wide, and placed together upon the disc 

 of the whirling-table, from which the cord has been removed, the 

 larger disc with the holes being nearest to the disc of the whirling- 

 table. The frame of the whirling-table is now, with the help of the 

 left hand and the left arm, placed upright before a mirror which is 

 fixed to a wall, the eye is applied to one of the holes, and the disc 

 turned with the right hand. 



The eye, as has been seen in the case of accidental 

 colours, is liable to a class of deceptions which are 

 usually called optical delusions. The combination of 

 lines represented in fig. 300 is an illustration of such a 



7 



v '/ 



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FIG. 300. 



delusion. The thick black lines are throughout 

 parallel, that is, at equal distances from one another, 

 as may easily be ascertained by means of a pair of com- 

 passes. Nevertheless, they appear to approach one 

 another at the top arid bottom alternately. The delu- 

 sion is caused by the presence of the thin cross-lines, 

 as may be seen by holding the page horizontally in a 

 line with the eye}, and looking along it. In this posi- 



