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XV. On a new Case of the Interference of Light. 
By the Rev. Baden Powell^ M.A., F.R.S., F.R.A.S., F.G.S., 
Savilian Professor of Geometry in the University of Oxford. 
Received April 6, — Read May 18, 1848. 
(1.) In the state of advance at which the theory of light has now arrived, a single 
case of interference directly explicable by the ordinary principles of undulations, 
even though occurring under new conditions, could hardly be deemed of sufficient 
importance to form the subject of a separate communication to the Royal So- 
ciety. 
But in the present instance though the case presented, in its more general features, 
is easily accounted for on the acknowledged doctrine of interference and retardation, 
yet it offers many particulars in its details, in reference to which such explanation is, 
at least, not equally obvious : while some points require for their elucidation investi- 
gations of a more extended character. 
Again, it is found to be a case which by no means stands isolated, but offers ana- 
logies with other classes of phenomena which have excited considerable interest and 
discussion, especially with regard to what has been termed, perhaps improperly, a 
“ polarity” in the prismatic rays,' — a new instance of which is here exhibited. In 
these respects, then, I trust the subject may not appear unworthy of the notice of the 
Royal Society. 
Having arrived at the primary results in July 1847, I communicated them to my 
friend Mr. G. G. Stokes of Pembroke College, Cambridge, who has gone extensively 
into the whole theory of these and other allied cases, and has afforded me much valu- 
able aid in the investigation. In the present paper I propose only to describe my 
own experiments, with the general application of the undulatory theory to the expla- 
nation of them, supported by some numerical comparisons. 
(2.) The main experiment is as follows : — in a hollow glass prism, or rather trough, 
containing some highly refractive and dispersive liquid, such as oil of sassafras, 
anise, or cassia, a plate of glass is inserted with its lower edge parallel to the edge of 
the prism, and so that its plane nearly bisects the angle of the prism, while it extends 
only through the upper half of the liquid, leaving the lower, or thinner part, clear 
(see fig. 1). Light being admitted through a narrow horizontal slit in the usual 
manner, the spectrum thus formed is seen crossed hy a number of dark bands parallel to 
the slit or edge of the prism. 
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