UPON THE PROPERTIES OF LIGHT. 
245 
Sines. 
Secants. 
Real value of y. 
Hyperbolic 
value. 
20 
35 
3^ 
65 
85 
^ 3 
1 1 1 
^ TT 
85 
I07i 
H 
H 
195 
240 
''12 
The unit here is -^oth of an inch. 
It is plain that this agrees nearly with the conic hyperbola, but in no respect with 
a straight line ; and upon calculating what effect the approach of P to C would have 
had, nothing could be more at variance with these numbers. But 
Exp. 2 . All doubt on this head is removed by making Pthe fixed point, and moving 
the first edge A nearer or further from it. In this experiment, the disturbing cause, 
arising from the varying distance from the chart, is entirely removed ; and it is uni- 
formly found that the decrease in the force varies notwithstanding with the increase 
of the distance. I have here only given the measures by way of illustration, and not 
in order to prove what the locus of?/ (or P) is, or, in other words, what the value of 
m is. 
Exp. 3. When one plate with a rectilinear edge is placed in the rays, and a second 
such plate is placed at any distance between it and the chart, the fringes are of equal 
breadth throughout their length, and all equally removed from the direct rays, 
each point of the second edge being at the same distance from the corresponding 
point of the first. But let the second plate be placed at an angle with the first, and 
the fringes are very different. It is better to let the second be parallel to the chart, 
and to incline the first; for thus the different points of the fringes are at the same 
distance from the edge which bends the disposed rays. In fig. 13, B is the second 
plate, parallel to the chart C ; A is the first plate ; all the points of B, from D to E, 
are equidistant from C ; therefore nothing can be ascribed to the divergence of the 
bent rays. B bends the rays disposed by A at different distances DD' and EE' from 
the point of disposition. The fringe is now of various breadths from dd! to e, the 
broadest part being that answering to the smallest distance of D, the point of flexion, 
from D' the point of disposition; the narrowest part, e, answering to EE', or the greatest 
distance of the point of flexion from the point of disposition. Moreover, the whole 
fringe is now inclined ; it is in the form of a curve from dd to e, and the broad part 
dd, formed by the flexion nearest the disposition, is furthest removed from the direct 
rays ; the narrowest part, e, is nearest these direct rays. It is thus quite clear that the 
flexion by B is in some inverse proportion to the distance at which the rays are bent 
by B from the point where they were disposed by A. I repeatedly examined the 
curve de, and found it certainly to be the conic hyperbola. Therefore m=l, and the 
equation to the force of disposition is 3 /=^- 
• 
In order to ascertain the value of m, I was not satisfied with ordinary admeasure- 
ments, but had an instrument made of great accuracy and even delicacy. It con- 
