point appears, and the phenomenon, as described at first, is complete.
438.
The relative size of the concentric circles and their intervals depends on the form of the glasses which are pressed together.
439.
We remarked above, that the coloured centre is, in fact, an undeveloped circle. It is, however, often found, on the slightest pressure, that several undeveloped circles exist there, as it were, in the germ; these can be successively developed before the eye of the observer.
440.
The regularity of these rings is owing to the form of the convex glasses, and the diameter of the coloured appearance depends on the greater or lesser section of a circle on which a lens is polished. We easily conclude from this, that by pressing plane glasses together, irregular appearances only will be produced; the colours, in fact, undulate like watered silks, and spread from the point of pressure in all directions. Yet, the phenomenon as thus exhibited is much more splendid than in the former instance, and cannot fail to strike every spectator. If we make the experiment in this mode, we shall distinctly see, as in the other case, that, on a slight pressure, the green and red waves appear; on a stronger, stripes of blue, red, and yellow, become detached. At first, the outer sides of these stripes touch; on increased pressure they are separated by a silver white space.
441.
Before we proceed to a further description of this phenomenon, we may point out the most convenient mode of exhibiting it. Place a