TRANSACTIONS OF THE SECTIONS. 
OOO 
On the Modification of the Interference of two Pencils of ho¬ 
mogeneous Light, produced by causing them to pass through 
a prism of glass; and on the Phenomena which then take 
place, with reference to the velocity of light in its passage 
through refracting substances. By Richard Potter, Jun. 
The principal part of this paper was occupied with mathema¬ 
tical investigations, relating to certain peculiarities which Mr. 
Potter detected, whilst repeating, in a different mode, an expe¬ 
riment first proposed by Professor Powell. The experiment 
consists in placing a prism of glass in the direction of two pen¬ 
cils of light, which produce bright and dark bands by interfe¬ 
rence, and then examining the light which has passed through 
the prism with air eye-lens. These pencils are mostly readily ob¬ 
tained by causing the rays which diverge from the image of the 
sun given by a lens of short focus, to fall upon two mirrors, 
very slightly inclined to each other. When the prism is placed 
in the direction of the interfering rays, the interference is not 
entirely prevented, but takes place between rays which have 
passed at a greater distance from the angle of the prism, than 
those which would have interfered if the prism had not been 
interposed. When the prism is small, or the overlapping of 
the pencils inconsiderable, there is a small distance from the 
prism, beyond which all appearance of interference ceases, on 
account that, the further from the prism is the position where 
the pencils are viewed with the lens, the further from the angle 
of the prism do the rays pass which interfere, until at last, from 
the causes mentioned, the effect ceases to take place. 
By altering the incidence on the prism, the breadth of the 
bands is also affected : from the angle of minimum deviation, 
towards a perpendicular incidence on the first surface, the 
bands become narrower and narrower, and on the opposite side 
of the same angle they become in the same manner broader. 
To find the central points of interference, according to any 
particular hypothesis on the velocity with which the light has 
traversed the prism, requires the previous consideration of three 
distinct questions, namely, first,—the positions of the secondary 
images of the original luminous point, or the centres of divergence 
of the rays after the two refractions : secondly,—the simulta¬ 
neous positions of the luminiferous surfaces : and thirdly,—the 
figures of the curves of the principal section of these surfaces, 
the plane of this section being common to both pencils. 
Having determined these questions on the supposition, first, 
