J. M. Moore, 
70 
center of the slot was a brass rod R, lcm in diameter, supported at 
either end by small blocks 2cm high. Upon this rod was a small 
brass cylinder D, closely fitting but free to move, which acted as a 
carriage for the second bead B supported similarly to the other two. 
It carried an index I which extended over a scale graduated in both 
directions from the center of the distance between the two beads. 
This carriage was moved by an endless cord passing over pulleys at 
the ends of the board. It also passed over a wheel W, whose axle 
was supported by two blocks under the table at a short distance 
from the front end. By turning the protruding axle the observer 
could place the middle bead wherever he desired. By pushing the 
lever L the first bead A could be raised and by pulling it the third 
bead C could be raised. This was done with the same hand which 
turned the roller for the second bead. From the rod F on the lower 
end of the second bead, a cord passed up behind and over the index 
rod I of the carriage, then through the slot S, under the table to a 
screw-eye in the front. This was used by the left hand and was 
kept stretched by a small weight. 
The beads were hidden from the observer by a blackened upright 
board E. In this a slot 2ram wide and llom long was cut, at the 
height which the beads would have when raised. To prevent the 
eyes from looking down further on the third bead than on the other 
two, by virtue of the visual angle covering more distance, a black 
tin apron G was extended 150m out from the slot. The head was 
kept in the same position by putting the nose at a definite place 
each time. 
The apparatus was placed on a large table and was raised or 
lowered so that when the observer was sitting erect, the eyes would 
be opposite the slot. The beads when raised were in the direct line 
of vision from the center of the slot. The nose having been put 
upon this middle point, each bead formed the vertex of an isosceles 
triangle made by the line of sight of the two eyes. Ko movements 
were necessary except those of accommodation and of symmetrical 
convergence. 
Method of experiment. 
The experiment began each time with the carriage of the second 
bead placed at the end of the slot nearest the observer. All bea 
were out of sight. By a push of the lever the first bead was raise.> 
its position was noticed and it was allowed to fall. By the cord 
the left hand the second bead was raised and dropped. Then by