the Cause of coloured concentric Rings. 
26 1 
3 6. Cylindrical and spherical Surfaces combined produce coloured 
elliptical Rings. 
The theory which suggests to us that the particular figure 
of every prismatic appearance between glasses depends on 
the curvature of the surfaces which are in contact, is still 
farther confirmed when spherical and cylindrical curves are 
applied to each other ; for these, accordingly, should give el- 
liptical rings ; and when I tried the experiment, by laying a 
26-inch double convex lens upon the cylindrical surface of my 
plate of glass, it produced a coloured elliptical central part, en- 
compassed with gradually vanishing rings of the same figure. 
By changing the focal length of the lens, I could alter the pro- 
portion of the conjugate to the transverse axes of these ellip- 
tical rings at pleasure. A lens of 55 inches gave ellipses that 
were much flattened, and one of 5 inches gave them nearly 
circular. 
37. Irregular Curves produce irregular Figures. 
The modifying power of surfaces may be further estab- 
lished by such as have no regular figure ; for these ought to 
give irregular prismatic phenomena, and this was fully proved 
by the following experiment. 
I took a large piece of mica which had a very glossy but 
irregular surface, and when a 34-inch double convex lens was 
placed upon a small ridge of it, several pretty straight streaks 
might be seen, but wherever the ridge was waving the streaks 
were following the same direction. In some places the mica 
gave irregular, coloured arcs, that were concave to some dis- 
tant centre ; and in others, the various contorted figures, that 
mdcccix. M m 
