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MR. STOKES ON TFIE THEORY OF CERTAIN 
Section L — Explanation of the formation of the hands on the imperfect theory of 
Interferences. Mode of calculating the number of hands seen in a given part of the 
spectrum. 
1. The phenomenon of which it is the principal object of the following paper to 
investigate the theory, is briefly as follows. Light introduced into a room through 
a horizontal slit is allowed to pass through a hollow glass prism containing fluid, 
with its refracting edge horizontal, and the spectrum is viewed through a small tele- 
scope with its object-glass close to the prism. On inserting into the fluid a trans- 
parent plate with its lower edge horizontal, the spectrum is seen traversed from end 
to end by very numerous dark bands, which are parallel to the fixed lines. Under 
favorable circumstances the dark bands are intensely black ; but in certain cases, 
to be considered presently, no bands whatsoever are seen. When the plate is cut 
from a doubly refracting’ crystal, there are in general two systems of bands seen 
together ; and when the light is analysed each system disappears in turn at every 
quarter revolution of the analyser. 
2. It is not difficult to see that the theory of these bands must be almost identical 
with that of the bands described by Sir David Brewster in the Report of the Seventh 
Meeting of the British Association, and elsewhere, and explained by Mr. Airy in the 
first Part of the Philosophical Transactions for 1841. To make this apparent, con- 
ceive an eye to view a spectrum through a small glass vessel with parallel faces filled 
with fluid. The vessel would not alter the appearance of the spectrum. Now con- 
ceive a transparent plate bounded by parallel surfaces inserted into the fluid, the plane 
of the plate being perpendicular to the axis of the eye, and its edge parallel to the 
fixed lines of the spectrum, and opposite to the centre of the pupil. Then we should 
have bands of the same nature as those described by Sir David Brewster, the only 
difference being that in the present case the retardation on which the existence of 
the bands depends is the difference of the retardations due to the plate itself, and to 
a plate of equal thickness of the fluid, instead of the absolute retardation of the plate, 
or more strictly, the difference of retardations of the solid plate and of a plate of 
equal thickness of air, contained between the produced parts of the bounding planes 
of the solid plate. In Professor Powell’s experiment the fluid fills the double office 
of the fluid in the glass vessel and of the prism producing the spectrum in the ima- 
ginary experiment just described. 
It might be expected that the remarkable polarity discovered by Sir David Brew- 
ster in the bands which he has described, would also be exhibited with Professor 
Powell’s apparatus. This anticipation is confirmed by experiment. With the 
arrangement of the apparatus already mentioned, it was found that with certain 
pairs of media, one being the fluid and the other the retarding plate, no bands were 
visible. These media were made to exhibit bands by using fluid enough to cover the 
plate to a certain depth, and stopping by a screen the light which would otherwise 
have passed through the thin end of the prism underneath the plate. 
