214 
PROFESSOR POWELL ON A NEW CASE OF 
(3.) I have tried various combinations of oils and other media with plates of dif- 
ferent thicknesses, both of glass and of other transparent substances. In these dif- 
ferent instances some remarkable distinctions are exhibited. In some cases the bands 
are sensibly equidistant, in others increasing in number and fineness towards one 
Fig. 1. Fig. 2. Position 1. Fig. 3. Position 2. 
Arrangement I. 
end of the spectrum : in most cases extending throughout, but in some deficient at 
one part. 
(4.) With the same oil or medium, if the plate exceed a certain thickness, the 
bands become too numerous and fine to be seen : if less than a certain other limit, 
they become too few, broad and faint : and for some intermediate thickness they 
appear most vivid and distinct. 
(5.) With plates and media of different refractive and dispersive powers as well as 
different thicknesses, changes in the number and vividness of the bands take place, 
as well as in the limits of their visibility, in a manner evidently dependent on the 
thickness and relative refractions jointly ; though not in any such obvious relation 
as can be stated by a simple experimental law. 
(6.) It is not necessary that the plate should have precisely the vertical position 
just described ; if inclined either way, even to being in contact with either side of the 
prism, the bands are still seen ; but they undergo a slight shifting downwards as the 
plate is inclined, and are perhaps less vivid (see figs. 2, 3). 
(7.) Some combinations of a medium and a plate, such as 
glass with oils of turpentine or angelica, or water, &c., give no 
bands ivith this arrangement. But Mr. Stokes pointed out 
from theory, that in these cases bands might be expected to 
appear with a reverse arrangement ; that is, by placing a 
narrow slip of glass, &c. to intercept the thinner part of the 
prism, leaving the upper or thicker part clear, and of course 
cutting oft' any portion of light, which might otherwise pass 
below the plate (see fig. 4, where ^ is a small screen for intercepting the light 
below). 
Fig. 4. 
