TRANSACTIONS OF THE SECTIONS. 
557 
tion for the eye being given, and a quadrant round the pivot 
graduated, these inclinations are easily found, and the intensi¬ 
ties of the reflections are then to be calculated by the formula 
which Mr. Potter discovered from photometrical measurements, 
and published in the Edinburgh Journal of Science. 
The apparatus producing the rings with homogeneous light 
being conveniently attached to the photometer, he compared 
them with two narrow stripes on the glasses, and found that 
when a green light, nearly homogeneous, produced by a solu¬ 
tion of arsenite of copper in diluted muriatic acid, was employed, 
the rings were represented in the photometer, when the intensity 
of the light in the glass representing the dark ring, was to the 
intensity in that representing the bright one, as 1* to about 2*48. 
With a very pure red light, produced by a solution of iodine in 
hydriodic acid, the difference was much greater, and the in¬ 
tensity of the light in the dark ring to that in the bright ring 
was found to be nearly as 1* to 3*5. The pasteboard surface 
was brought as nearly to the same colours as possible, to enable 
the eye to judge with greater accuracy. 
Dr. Young and Sir John Herschel have each given formulas 
for the difference of the intensities according to the undulatory 
theory, which, excepting that the latter has used certain ap¬ 
proximations, coincide. 
From the formula of Sir J. Herschel, Mr. Potter finds the 
ratio of the intensities for pure homogeneous light should, if 
that theory were correct, be as 1* to 1*1538, a ratio widely dif¬ 
ferent from 1 to 3*5, as he found it to be nearly in fact. 
Finding his photometer applicable to comparing the reflec¬ 
tions of crown glass with that of other substances, he examined 
by it the reflective powers of diamond, mica, rock crystal, 
selenite, Iceland spar, emerald, and amethyst. Diamond he 
found to reflect at 10° incidence, about 9*4 of every 100 rays 
incident. By the formula according to the undulatory theory, 
namely, 18*36 rays of every 100 should have been 
reflected at a perpendicular incidence, in lieu of about half 
that number which are in reality reflected. 
With the other substances, the reflections by mica, rock 
crystal, Iceland spar, emerald, and amethyst, were rather 
higher than that of crown glass; the reflection by a recent 
surface of selenite was so nearly that of crown glass, that he 
found it impossible to state whether it was in reality higher or 
lower. 
