200 Mr. S. Rowley on a New Theory of Vision. 



A still better instrument is a small wooden cross consisting 

 of a slender bar and sliding transverse piece, which may be any 

 slip of wood (as a piece of common lath) notched at the centre 

 so as to receive the longitudinal bar in its full thickness at right 

 angles, and also capable of being shifted and fixed by wedging 

 at any point. 



For all experiments in which the distance of the remoter of 

 the two stations designated A and B, tigs. 1 and 2, is not greater 

 than 18 inches, the length of the bar should be 18 inches, and 

 that of the transverse piece 12 inches. 



To apply this instrument, instead of the window-bar and rulers 

 in the first of the examples taken, fix the transverse piece at the 

 distance of 17 inches from one end of -p.^ . 



the shaft, and laying the cross on - 1 



o 



f 1 1 , 



T' D 



table with its level side upward, draw r 



on the upper side of the shaft length- p x B 

 wise the bisecting line N 0, and length- 

 wise on the upper side of the transverse 

 piece the bisecting line PL. At B, 

 the point of intersection of these lines, 

 set a pin, and at the point A, 12 inches 

 from the point B, in the line N 0, also 

 one. At the points T, T ; in the line 

 P L, each distant 2^ inches from B, 

 set two other pins, and at D, 4 T 9 inches 

 from B, another. Then placing the cross th 



so as to lie with the extremity N of the 



shaft against the upper part of the nose, and its upper surface a 

 little below and parallel to the plane of the axes, and keeping 

 the head fixed, look with each eye in rapid succession at its image 

 of the pin at A. If the right eye's image of the pin at T, and 

 the left eye's of that at T', be seen in these directions of the 

 axes, the pins at A and B are 6 and 18 inches distant from the 

 middle point of the line joining the centres of the eyes. But if 

 they should fall within or without the axes thus directed, then 

 the transverse piece, together with the pins at A and B, 12 

 inches apart, must be shifted slightly forward or backward until 

 the requisite position is found. 



Holding this instrument in the same position with respect to 

 the eyes as that just above given to it for ascertaining distance, 

 turn it toward a strongly illuminated white wall or sheet of white 

 paper, or (which is better) the clear sky. If now the axes be 

 fixed upon the images of the pin at A, the right eye's image of 

 the pin at B and the left eye's of that at D will be seen lying in 

 the same line of no variation, the former a little beyond the 

 latter. But if, having transferred the transverse piece to the 



