Oct. 3, 18B4.] 



♦ KNOWLEDGE ♦ 



279 



and coalesce. This may be eflFected in the case of drawings 

 or photographs, after a little practice, by squinting. Fig. 28 

 ■will illustrate this. It represents a truncated cone, or cone 

 with its sharp end cut otT, as seen by tbe right and left 

 eyes re.spectively. If now the reader will get a strip of 

 cardboard (preferably of a dead black) about a foot long, 

 and place it on end half way between L .and R in our 

 iigure, with the other end against liis forehead and the root 

 of his nose, so that R is visible only to the riglit eye and L 

 to the left one ; then, on looking, so to speak, at a point 

 far on the other side of this page, the two images will be 

 seen to run suddenly into one, and a startlingly solid little 

 cone will seemingly rise out of the paper. As we have said 

 above, the little circles require greater convergence to make 

 them coalesce than the larger ones, and so are felt to be 

 nearer to the eye, the result being the illusive conviction 

 that we are actually regarding an objective skeleton cone. 

 A careful examination of a photographed stereoscope-slide 



I 



M 



R 



Fig. 29. 



Fig. 30. 



will show the dissimilarity of the images of objects in the 

 immediate foreground, and their absolute identity in distant 

 ones. What we have effected by squinting, the lenticular 

 form of stereoscope devised by Sir David Brewster (with 

 which, at all events in its outward form, probably every 

 reader is familiar) does in an even more simple and con- 

 venient manner. If the reader will turn back to page 46 

 and trace the course of a sheaf of rays through a convex 

 lens, he will be prepared to understand the principle of 

 this stereoscope at once. For we simply take a convex 

 lens, like A C in Fig. 19, cut it in half across a diameter 

 B C, and so set the two halves in a frame, the width of the 

 eyes apart, that the sides A and B shall be inward, or next 

 the nose. The left-hand part of Fig. 29 represents the lens 

 as \'iewed from the front before its halves have been 

 separated. The middle one shows the two halves in plan 

 m the position they occupy as the eye-pieces in a stereoscope. 

 The right-hand diagram exhibits them in section, and shows 



how they act. Turning to Fig. 18, p. 40, for an illii.stra 

 tion of the course of a ray of light through a prism, it 

 will be seen at once in Fig. 29 that a ray coming from the 

 lef1>hand picture L will be refracted hy the prism A C D to 

 /, and, of course, be referred by the left eye to M ; while 

 the ray from the right-hand picture R will, in like man- 

 ner, be refracted to r by the prism BCD and equally re- 

 ferred to M by the right eye, the illusive impression of 

 solidity from the coalescence of the two slightly dissimilar 

 pictures thus seemingly both emanating from a single object 

 at M being perfect. It is needless to pursue this branch 

 of our subject here, enough having been said to show 

 how it is that we derive the impression of solidity from 

 binocular vision, and how perfectly the sense of sight may 

 be deceived by the presentation to the two eyes of accurate 

 simulacra of the images which they would receive from a 

 material object. 



A ridiculous deception incident on our seeing two images 

 was described by Mr. T. Foster 

 on p. 244 of our first volume, to 

 which the student is referred for 

 an experiment as instructive as 

 it is absurd in its effect. An- 

 other peculiarity of vision, or 

 properly of the retina and optic 

 nerve, is at the bottom of 

 several familiar optical pheno- 

 mena and visual deceptions. 

 We refer to the retention on 

 the retina, for something like 

 the tenth of a second, of the 

 impression of an object, after 

 that object has passed away or 

 been otherwise hidden. Ob- 

 serve particularly that it is by 

 no means necessary that any 

 given object should remain be- 

 fore the eye for such an interval 

 for its original perception. So- 

 far from this being the case, 

 we distinctly see a flash of 

 lightning, whose duration was 

 shown by Wheatstone to be 

 less than a single thousandth 

 of a second. Once seen, how- 

 ever, the image persists for 

 the time first specified. Pro- 

 bably the bestknown and 

 commonest results of this is 

 that when we wink we do not 

 lose sight of the objects at 

 which we may happen to be looking ; their images con- 

 tinuing to impress the retina for the very short fnterval 

 during which the eyes are closed in the act. It is, of 

 course, this temporary persistence of the images of external 

 objects which is an integral element in the explanation of 

 the illusions we experience with such toys as the magic 

 Wheel (Knowledge, vol. i., pp. 198, 199, and 247), tbe 

 Strobic Circles (Id, p. 422), and the so-called Zoetrope (or 

 Wheel of Life), in which such figures as those shown on 

 pp. 199 and 247 of our first volume are placed round tbe 

 interior of a cylinder, in which slits are cut, and the figures 

 viewed through these slits from the outside as the cylinder 

 rotates, thus dispensing with a looking-glass. The boy's 

 experiment of whirling a lighted stick about, and producing 

 the semblance of a, continuously-curved line of light also- 

 depends upon this property of the eye, wbieh may be 

 further illustrated by a very simple contrivance (shown in 

 Fig. 30), popular in the days of our fathers, and known 



