stablished the laws of the appearances presented. I will now endeavor
to explain the general principle by which this phenomenon is utilized in
the testing of plane surfaces. Suppose that we place on the lower plate,
lenses of constantly increasing curvature until that curvature becomes
nil, or in other words a true plane. The rings of color will constantly
increase in width as the curvature of the lens increases, until at last
one color alone is seen over the whole surface, provided, however, the
same angle of observation be maintained, and provided further that the
film of air between the glasses is of absolutely the same relative
thickness throughout. I say the film of air, for I presume that it would
be utterly impossible to exclude particles of dust so that absolute
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 discuss