But the greatest discrepancy is seen in Figs. 28 and 29, the middle line in Fig. 29 appearing to be much longer than in Fig. 28. Careful measurement will show that they are both of precisely the same length, the apparent difference being due to the arrangement of the divergent lines at the ends. [Illustration: Fig. 28.] [Illustration: Fig. 29.] [Illustration: Fig. 30.] Converging lines have a curious effect upon apparent size. Thus in Fig. 30 we have a wall and three posts, and if asked which of the posts was the highest, most persons would name C, but measurement will show that A is the highest and that C is the shortest. [Illustration: Fig. 31.] A still more striking effect is produced in two parallel lines by crossing them with a series of oblique lines as seen in Figs. 31 and 32. In Fig. 31 the horizontal lines seem to be much closer at the right-hand ends than at the left, but accurate measurement will show that they are strictly parallel. By changing the direction of the oblique lines, as shown in Fig. 32, the horizontal lines appear to be crooked although they are perfectly straight. [Illustration: Fig. 32.] All these curious illusions are, however, far surpassed by an experiment which we will now proceed to describe. FOOTNOTES: [9] The old and generally recognized list of the senses is as follows: Sight, Hearing, Smell, Taste, and Touch. This is the list enumerated by John Bunyan in his famous work, "The Holie Warre." It has, however, been pointed out that the sense which enables us to recognize heat is not quite the same as that of touch and modern physiologists have therefore set apart, as a distinct sense, the power by which we recognize heat. The same had been previously done in the case of the sense of Muscular Resistance but, as the author of "The Natural History of Hell" says, "when we differentiate the 'Sense of Heat,' and the 'Sense of Resistance' from the Sense of Touch, we may set up new signposts, but we do not open up any new 'gateways', things still remain as they were of old, and every messenger from the material world around us must enter the ivory palace of the skull through one of the old and well-known ways." OBJECTS APPARENTLY SEEN THROUGH A HOLE IN THE HAND The following curious experiment always excites surprise, and as I have met with very few persons who have ever heard of it, I republish it from "The Young Scientist," for November, 1880. It throws a good deal of light upon the facts connected with vision. [Illustration: Fig. 33.]