contact could take place. Early physicists maintained that absolute
molecular contact was impossible, and that the central separation of the
glasses in Newton's experiment was 1/250,000 of an inch, but Sir Wm.
Thomson has shown that the separation is caused by shreds or particles of
dust. However, if this separation is equal throughout, we have the
phenomena as described; but if the dust particles are thicker under one
side than the other, our phenomena will change to broad parallel bands as
in Fig. 8, the broader the bands the nearer the absolute parallelism of
the plates. In Fig. 7 let _a_ and _b_ represent the two plates we are
testing. Rays of white light, _c_, falling upon the upper surface of plate
_a_, are partially reflected off in the direction of rays _d_, but as
these rays do not concern us now, I have not sketched them. Part of the
light passes on through the upper plate, where it is bent out of its
course somewhat, and, falling upon the _lower_ surface of the upper plate,
some of this light is again reflected toward the eye at _d_. As some of
the light passes through the upper plate, and, passing through the film of
air between the plates, falling on the upper surface of the _lower_ one,
this in turn is reflected; but as the light that falls on this surface has
had to traverse the film of air _twice_, it is retarded by a certain
number of half or whole wave-lengths, and the beautiful phenomena of
interference take place, some of the colors of white light being
obliterated, while others come to the eye. When the position of the eye
changes, the color is seen to change. I have not time to dwell further on
this part of my subject, which is discussed in most advanced works on
physics, and especially well described in Dr. Eugene Lommel's work on "The
Nature of Light." I remarked that if the two surfaces were perfectly
_plane_, there would be one color seen, or else colors of the first or
second order would arrange themselves in broad parallel bands, but this
would also take place in plates of slight curvature, for the requirement
is, as I said, a film of air of equal thickness throughout. You can see at