316 Sir D. Brewster on the Ktioxvledge of 



and meeting had^ 'Lhe at d and c, d being the binocular centre 

 when we look at it through a and b, and e the binocular centre 

 when we look at it through b and c. Now, the right eye R 

 sees the opening b at d, and the left eye L sees the opening a 

 at d, hence the image at d consists of the similar images of a 

 and b united. In like manner e consists of b and c united, 

 and so on with all the rest, so that the observer at LR no 

 longer sees the real pattern M N, but a suspended image of 

 it at mn, three inches behind M N. If the observer now ap- 

 proaches M N, the image mn will approach to him, and if he 

 recedes, m7i will recede, being \^ inch distant from MN 

 when the observer is six inches from M N, and twelve inches 

 from M N when he is forty-eight inches from M N, the image 

 vm moving from M N with a velocity one-fourth of that with 

 which the observer recedes. These two velocities are in the 



ratio or u to p — -, 



Resuming the position in the figure where the observer is 

 twelve inches distant from M N, let us consider the important 

 results to which this experiment cannot fail to lead us. If the 

 observer, with his eyes at L R, grnsp the cane bottom or 

 pattern at M N, as shown in fig. 4, his thumbs pressing upon 

 M N, and his fingers trying to grasp mn, he will ihen feel 

 xohat he does not see, and see what he does not feel ! The real 

 pattern is absolutely invisible at M N, and stands fixed at m n. 

 The fingers may be passed through and through — now seen 

 on this side of it, now in the middle of it, and now on the 

 other side of it. If we next place the palms of each hand 

 upon M N, feeling it all over, the result will be the same. No 

 knowledge derived from touch — no measurement of real di- 

 stances — no actual demonstration from previous or subsequent 

 vision, that there is a real solid body at M N, and nothing at 

 all at 11171, will remove or shake the infallible conviction of the 

 sense of sight that the object is at mn, and that d L or </ R is 

 its real distance from the observer. If the binocular centre 

 be now drawn back to M N, the image seen will disappear, 

 and the real object be seen at M N. If it be brought still 

 further back to^^ the object M N will again disappear, and 

 will be seen at /x v, as described in a former part of this paper. 



In making these experiments, the observer cannot fail to be 

 struck with the remarkable fact, that though the openings at 

 M N, mn, and ju, v have all the same angular magnitude, that 

 is, subtend the same angle at the eye, viz. d L e, rf R e, yet 

 those at inn appear larger than those at M N, and those at j«.v 

 smaller. If we cause the image inn to recede and ju,v to ap- 

 proach, the figures in m n will invariably increase as they 



