174 On a New Case of Interference 
tered ; and that the only effect of the acceleration is to push the entire system from 
the edge, the amount of the shifting being equal to } A cotan a. 
In order to submit these results to the test of trial, I employed the apparatus 
consisting of two moveable metallic plates, which is of so much use in experiments of 
interference. The plates being closed, so as to form a narrow horizontal aperture, the 
flame of a lamp was placed behind ; and the light thus diverging from the aperture 
was received, at the distance of about three feet, on a piece of black glass truly 
polished, and also horizontal. This reflector was then adjusted, so that its plane 
might pass a little below the aperture ; or, in other words, that the light might be incident 
upon it at an angle of nearly 90°. It is evident, then, that the light thus obliquely re- 
flected will meet the direct light diverging from the aperture under a very small angle, 
and with a difference in the lengths of their paths which is capable of indefinite dimi- - 
nution. The two lights, therefore, are in a condition to interfere; and I found, ac- 
cordingly, that when they were received upon an eyepiece, placed at a short distance 
from the reflector, a very beautiful system of bands was visible, in every respect simi- 
lar to one-half of the system formed by the two mirrors, in FresNeL’s experiment. 
The first band was a bright one, and colourless. This was succeeded by a very 
sharply defined black band; then followed a coloured bright band, and so on alter- 
nately. Under favorable circumstances I could easily count seven alternations ; the 
breadth of the bands being, as far as the eye could judge, the same throughout’ the 
series, and increasing with the obliquity of the reflected beam. The first dark band 
was of intense blackness; but the darkness of the succeeding bands was less intense, 
as they were of higher orders; and after three or four orders, they were completely 
obliterated by the closing in of the bright bands. At the same time the coloration of 
the bright bands increased with the order of the band ; until, after six or seven alter- 
nations, the colours of different orders became superimposed, and the bands were thus 
lost in a diffused light of nearly uniform intensity. All these circumstances are simi- 
lar to those observed in FresNet’s experiment, and correspond exactly with the results 
of theory. 
These bands are most perfectly defined when the eyepiece is close to the reflector. 
Their breadth and coloration increased with the distance of the eyepiece, but re- 
mained of a finite and very sensible magnitude, when the latter was brought into 
actual contact with the edge—a circumstance whith distinguishes them altogether 
from the diffracted fringes formed on the boundary of the shadow. 
These fringes appear to me to possess some interest in a theoretical point of view, 
independently of that which attaches to them as illustrations of an important general 
law. Depending on the interference of two lights, one of which proceeds directly 
from the luminous origin, while the other has undergone reflexion, they would seem 
to afford the means of detecting any difference which might exist in their condition 
when they meet, and therefore of tracing the modifications produced by reflexion. 
