138 



EXPEBIHENTS AND INVESTIGATIONS 



fourth gives the value of y, supposing M N were a conic 

 hyperbola. 



The unit here is -gVth of an inch. 



It is plain that this agrees nearly with the conic hyper- 

 bola, 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 P 

 the fixed point, and moving the first edge A nearer or further 

 from it. Jn this experiment, the disturbing cause, arising 

 from the varying distance from the chart, is entirely re- 

 moved ; and it is uniformly found that the decrease in the 

 force varies notwithstanding with the increase of the dis- 

 tance. I have here only given the measures by way of 

 illustration, and not in order to prove what the locus of y 

 (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 ; 



