UPON THE PROPERTIES OF LIGHT. 
259 
graduated, F, by which the relative distances can always be determined of the plates 
A,C and B. A moves up and down upon H, B upon I, and C upon K ; each plate is 
moved up and down by rack and pinion D. The uprights also move along the groove 
G by rack and pinion E. 
PLATE XV. 
Is the instrument for ascertaining more nicely the effects of distance on disposi- 
tion. A is a plate with graduated edge ; it moves vertically on a pivot, and its angle 
with the horizontal line is measured by the quadrant E. A also moves horizontally, 
and its horizontal angle is measured by the quadrant K. B is another plate with 
graduated edge, moving in a groove D, by rack and pinion H, and along a graduated 
beam I. F is a fine micrometer, by which the distance of A above B, when A is hori- 
zontal, can always be measured to the greatest nicety by the circle F and the scale G. 
PLATE XVI. 
Is an instrument also for measuring the effect of the distance of the edges upon 
the disposing forces. C C C is a graduated beam, adjusted by the spirit-level, and 
on it moves the upright on which a plate A moves by micrometer screw E, so that 
the distance of A from the rays that pass along C C C after flexion by a plate fixed 
at one end of the beam, can be ascertained by the scale D. I have experimented with 
this, but I did not find it so easy to work by as the other apparatus. C C C is 
brought to an exact level by screws not noted in the drawing. 
2 L 2 
