258 
LORD brougham’s EXPERIMENTS AND OBSERVATIONS 
distance, and as this forms an inherent and essential property of tlie light itself, what 
is the result ? Plainly this, that the motion of light is quite uniform after flexion, and 
apparently before also. The flexion produces acceleration but only for an instant. 
If ss is the space through which the ray moves after entering the sphere of flexion, 
and V the velocity before it enters that sphere ; it moves after entering with a velocity 
=\/v‘^-\-Zdz, Z being the law of the bending force. Then this is greater than v; 
consequently there is an acceleration, though not very great ; but because y—-, if 
oc 
• s tds sdt (JL 
is the space, t the time, the force of acceleration is ^ X — ^ — ; h\\ty—~ shows that s 
d • s tds ~~’sdt 
is as #, else z/= - would be impossible ; therefore the accelerating force ^X— ^3 — = 0, 
and so it is shown there is no acceleration after the ray leaves the sphere of flexion. 
Description of the Instruments. 
PLATE XII. 
Is the instrument with two plates or edges. A, B, horizontal, D, C vertical ; the 
former moved by the screw E, which has also a micrometer for the distances on the 
scale G ; the latter, in like manner, moved by F, connected with micrometer and 
scale H. 
PLATE XIII. 
Is the instrument with four surfaces. AD, ad are two parallel plates, moving 
horizontally by a rack and pinion E. Each plate has an edge composed of four sur- 
faces ; A, a, a sharp edge or very narrow surface ; B, &, a flat surface ; C, c, a cylindri- 
cal surface of large radius of curvature, and so flat ; D, d,one of small radius, and so 
very convex ; this is represented on the figure by A' B' C' D' beside the other. Care 
is to be taken that AB CD and ab cdhe a perfectly straight line, made up of the 
sharp edge, the plane surface and the tangents to the two cylinders. H is a plate 
with a sharp and straight edge, oy, which can be brought by its handle F to come 
opposite to the compound edge abed, when it is desired to try the flexion by the 
latter, without another flexion by an opposite compound edge, but only with a flexion 
by a rectilinear simple edge. 
PLATE XIV. 
Is the instrument by which is tried the exyerimentum crucis on the action of the 
third edge, and also the experiments on the distances of the edges as affecting the 
disposing force. G is the groove in which the uprights H, I, K move. There is a scale 
