QT2 



SIR DAVID BREWSTER ON THE KNOWLEDGE OF 



From these formulae we have computed the following table, adapted to simi- 

 lar figures, whose centres are distant ^ an inch, 1, 1^, 2, and 2^ inches; but in 

 reference to the positive values of A and D, we may consider them as feet, 0-5 

 being in that case = 6 inches. 



Taking the case where D is 12 inches, and uniting the two nearest openings 

 where d is 0-5, let M N (Fig. 4, PL 17) be a section of the transparent pattern, L, R 

 the left and right eyes, L a d, L b c lines drawn through the centres of two of the 

 open figures a b, and R b d, Rce lines drawn through the centre of b and c, and 

 meeting Lad, Lie aid and e, 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 ob- 

 server at L R no longer sees the real pattern M N, but a suspended image of it at 

 m n, three inches behind M N. If the observer now approaches M N, the image 

 m n will approach to him, and if he recedes, m n will recede, being 1-J inches dis- 

 tant from M N when the observer is G inches from M N, and 12 inches from M N 

 when he is 48 inches from M N, the image m n moving from M N with a velocity 



of that with which the observer recedes. These two velocities are in the ratio 



n T\ i 



of D to 



Resuming the position in the figure where the observer is 12 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, grasp the cane bottom or 

 pattern at M N, as shewn in Fig. 4, pi. 17, his thumbs pressing upon M N, and his 

 fingers trying to grasp m n, he will then feel what 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. 



