THE INTERFERENCE OP LIGHT. 
217 
(17.) Hence Mr. Stokes illustrated the case by supposing the glass plate inserted 
in a vessel with parallel sides filled with the medium ; when if the spectrum formed 
by any prism were viewed through this combination, with the plate towards the 
thick part of the prism, the effect would 
be the same ; and if the prism were of the 
same substance, the cases would be iden- 
tical; the liquid prism in my experiment 
serving the double office of the prism and 
the flat vessel. (See fig. 5, where P is the 
plate and M the medium.) 
(18.) Another remark here offers itself: — 
If it should be considered that the theory is sufficiently established to give confidence 
in deductions from it, it may be applied to the inverse problem of finding the refractive 
indices of a plate, those of the medium, and the number of bands, being known. This 
may be important for many substances which occur in the form of plates, but cannot 
be cut into prisms. 
(19.) There remain also other features of the case to be accounted for, which, if 
slight in appearance, are yet not unimportant in a theoretical point of view ; such as 
relate to the changes in the vividness of the bands, especially as affected by the 
thickness or inclination of the plate, by enlarging or contracting the aperture, or 
breadth of the prism, — and some other points, — to which the simple interference- 
theory cannot apply. 
In all experiments of this kind it is now generally understood that there must 
be, theoretically at least, even if it be practically insignificant, another species of 
action concerned, dependent on the diffraction of the lens, whether that of the eye, 
or of the object-glass of the telescope, producing that diffusion,” as it has been 
termed, in the optical image, which, if of sensible amount, may influence the phe- 
nomena. 
The method of investigating this species of action in general is equally well under- 
stood, though in some parts the theory has been found susceptible of improvement. 
In the present instance such an investigation is necessary to include some of the 
peculiar modifications of the phenomena observed ; and this constitutes the subject 
of Mr. Stokes’s researches, which he will give in a separate form. 
The remainder of this paper is devoted to the details of the observations and the 
theoretical investigations. 
Theoretical Investigation. 
(20.) The intensity of the incident light being unity, and the direet vibration for 
any point in the spectrum being 
Fig. 5. 
