556 
SECOND REPORT — 1832 . 
that the velocity of light, in passing through transparent media, 
is inversely as their refractive indices, and then, that it is di¬ 
rectly as their refractive indices, and having aftewards calcu¬ 
lated the points where interference should have taken place ac¬ 
cording to each hypothesis, Mr. Potter inferred that neither 
one nor the other would account for the phenomena which he 
had observed. He concluded also that it is necessary to con¬ 
sider light as moving through the prism with even less velocity 
than the reciprocal of the refractive index of the glass, to ac¬ 
count for the actual phasnomena. 
On an Instrument for Photometry by comparison , and on some 
Applications of it to optical Phcenomena . By Richard 
Potter, Jun, 
When engaged in examining the phenomena of the colours 
of thin plates, in the form of what are generally denominated 
Newton’s rings, Mr. Potter was surprised to find the rings ap¬ 
pear so distinctly in the transmitted light, and particularly when 
homogeneous light was used. If, as is now generally allowed, 
these rings are produced by the interference of the light which 
has been twice reflected, at an incidence nearly perpendicular 
on the glass, the difference of the intensities of the dark and 
bright rings is much greater than can be accounted for on any 
mechanical theories of the intensity of light in interference. For 
if, as it is found to be very nearly by experiment, the reflection 
at a surface of common glass is about ^th, two reflections would 
give an intensity of about g^gth5 anc ^ the light causing interfe¬ 
rence would be only g^th part of the whole transmitted light,—a 
quantity of which the presence or absence is, in common circum¬ 
stances , far beyond the limits of detection by the eye. Hence 
he inferred the importance of determining the relative intensi¬ 
ties of the light in the dark and bright rings, and invented this 
photometer for the purpose. It consists of a rectangular piece 
of pasteboard, about 23f inches long and about 8f inches broad, 
set edgewise, in the form of a semicircle, on a rectangular piece 
of wood; in the centre is a pivot, round which turn two arms, 
carrying pieces of plane crown glass, blackened at their further 
surfaces. These pieces of glass are so mounted, as when they 
are moved with the arms round the pivot, to remain, like the 
pasteboard, perpendicular to the plane of the rectangular board. 
When the pasteboard is equally illuminated in every part, as on 
a misty or uniformly overclouded day, the intensities of the 
reflections of its surface seen in the glasses depend only on their 
inclinations to the directions of the visual rays. A fixed posi- 
