S36 Mr. R. Potter, Jun., on the Velocity with which 



ajul it carried up or clown, when turned, a piece of hard wood, 

 to which was attached the eye-lens with a needle-point in its 

 focus. By means of an index and a divided card, the motion 

 of the screw was read off to the 20()dth part of a revolution. 



The distance of the points a and b was determined by 

 the method used in a similar case by M. Fresnel, namely, by 

 placing a piece of tin-plate, so that a circular perforation in it 

 might be on the line ep, as in fig. 2. A series of coloured 

 rings round a centre, also coloured, but well defined, was in 



this manner seen for each luminous point a and b, and the 

 distance i i of these centres was very easily and accurately de- 

 termined with the screw carrying the eye-glass ; from which 

 the distance a b was easily deduced, when the distances e r 

 and rp had been measured. 



There are many points that require great attention in con- 

 ducting the experiment, with which I must not trespass longer 

 on the reader's patience, but will merely state that they arise 

 from the necessary smallness of the distance a b, the diffracted 

 fringes of the edges of the glasses hi, li, the peculiar adjust- 

 ments required for the mirrors, the positions of the mirrors 

 with respect to the original luminous point, and the positions 

 of the glasses with respect to the points e 9 fand a, b. 



The final adjusting of the apparatus often required all the 

 sun-light I could obtain, for one or two days before the ex- 

 periment could be proceeded with ; and this occurring every 

 time the apparatus was altered so as to experiment under dif- 

 ferent circumstances, prevented me, even during the long suc- 

 cession of fine weather we had this spring, from obtaining 

 more numerous measures. The alterations of the apparatus 

 were to produce different values for the distance ab, and the 

 proper places of the edges h i, I z, for the different angles be- 

 tween the glasses. For all the measures in which the glasses 

 were inclined to each other more than 10 degrees, the two sets 

 of fringes at e andy were seen, and afforded the means of 

 taking the distances of these points with great accuracy. In 

 the others, one set only could be seen at once, so that the lower 

 glass had to be brought up to a parallel position with the 

 upper one for every measure; the lower pencil passing in 



