S. Rowley on Vision. 161 
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 cen- 
ter so as to receive the longitudinal bar in its full thickness at 
right angles, and also capable of being shifted and fixed b 
wedging at any point. 
or all experiments in which the distance of the remoter of 
the two stations designated A and B, figs. 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 en 
of the shaft, and laying the cross on a y 
table with its level side upward, draw on 
the upper side of the shaft lengthwise TB) T’ D 
the bisecting line NO, and lengthwise 
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 NO, also one. At 
the points T, 'T’, in the line PL, each dis- 
tant 2:4, inches from B, set two other pins, 
and at D, 4, inches from B, another. N 
Then placing the cross so as to lie with 
the extremity N, of the shaft against the upper part of the 
hose, and its upper surface a little below and parallel to the 
Plane of the axes, and keeping the head fixed, look with each 
“ye m rapid succession at its image of the pin at A. If the 
night eye’s image of the pin at T, and the left eye’s of that at 
» De seen in these directions of the axes, the pins at A a 
B are 6 and 18 inches distant from the middle point of the 
- joining the centers 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 requi- 
Site position is found. 
sh tolding this instrument in the same position with respect to 
‘Yes as that just above given to it for ascertaining distance, _ 
_ tan it toward a strongly illuminated white wall or sheet of 
: the — same line of eon pre a 
