326 



SCIENTIFIC AMUSEMENTS. 



Fig. 168. Pseu- 

 doscope. 



hollow be looked at, it generally appears to be in relief, and vice 

 versa; some people, however, are unable to perceive this effect, 

 although perfectly able to distinguish the reversal 

 of position as regards right and left. 



Many curious effects and illusions can be pro- 

 duced by taking advantage of the fact that the 

 sense of vision under ordinary circumstances is based 

 on the action of the eye as an optical instrument 

 (Expt. 372), conjoined with the effect of habit and 

 previous notions derived from experience. Thus 

 most people have a pretty clear idea of what a length 

 of 10 inches represents on a sheet of paper, such as 

 a map placed in front of them. But if anyone put 

 a "chimneypot" hat just 10 inches high on his 

 head to be looked at, roughly speaking, level with 

 the eye, and the observer be asked to mark off on 

 the skirtingboard or wall of a room the height to which the 

 brim of the hat will reach when standing on the floor on its 

 crown, the judgments formed will in 

 most cases differ considerably from each 

 other and from the truth. Fig. 169 re- 

 presents a somewhat different case, 

 where, owing to the effect of custom 

 and habit in allowing for perspective in 

 looking at objects, the inclined lines, 

 although really parallel, do not seem to 

 be so. Similarly, in fig. 170 the lower 

 halves of the letters S and figures 8 



,.,. appear to most persons of the same size 



Fig. 169. Zollner s Lines. ag ^ upper halyeg . but Qn tuming ^ 



figure upside down it will be seen that the lower halves are really 



ssssssss 



L Fig. 170. 



Fig. 171, Black and White Circles. 



88888888 



larger. Again, in fig. 171 

 the white spot on a black 

 ground appears larger than 

 the black spot on a white 

 ground when viewed from 

 a short distance, although 

 both are really of the same 

 size. If the centre of fig. 

 172 be looked at from a 



