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 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 once that this condition could be obtained in a perfect convex surface fitting a perfect concave of the same radius.  Fortunately we have a check to guard against this error.  To produce a perfect plane, three surfaces must be worked together, unless we have a true plane to commence with; but to make this true plane by this method we must work three together, and if each one comes up to the demands of this most rigorous test, we may rest assured that we have attained a degree of accuracy almost beyond human conception.  Let me illustrate.  Suppose we have plates 1, 2, and 3, Fig. 11.  Suppose