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Rh Prof. Morley: The condition in which they appear when we count them may be described. Suppose our standard is adjusted so that its front mirror rigorously coincides with the image of the opaque mirror of the refractometer. This image becomes an immaterial plane of reference which we can move slowly till it coincides with the rear mirror of the standard, being throughout parallel to both. During this motion, interference fringes appear as alternate light and black circles, the one at the centre being a light or black spot of considerable size; the black or light circle outside the central spot moves inwards, taking its place, and the central spot vanishes; the other circles all move inwards at the same time. While the central black or white spot has become white or black, the immaterial reference plane has moved one quarter of a wave length; when it has again become black or white, the motion has been one half wave length. So if we count the returns of the blackness, we count the half wave lengths in the motion of our reference plane. The phenomenon is one very easy to count. I counted 4,500 wave lengths the first time I tried counting at all. If it were necessary a machine could be made to count them automatically, only needing the observer to verify the continuance of restoration of its adjustment during the process. It is as easy to count them as to count the black posts of a fence with a white background.

Mr. Warner: Can Professor Morley give us some familiar illustration of the magnitude of a wave length? He spoke the other day of a piece of split glass which would show all the colors of the rainbow.

Prof. Morley: Mr. Warner, I think, showed me a piece of glass cracked, but not broken apart. Looking at it in a proper light, a part of the light was reflected to the eye from the upper surface and a part from the lower surface of the fracture, and these two rays again united. We then got the colors of the rainbow due to interference. If you press the two parts enough to make one band of color take the place of the adjacent band of the same color, you have compressed the glass by one half wave length.

Mr. Force: You said that the temperature did not affect the measurements by this method; was that on account of your using a vacuum in counting wave lengths?

Prof. Morley: The wave length in a vacuum is the same at any temperature. It is necessary to have a good vacuum; but that is easy to obtain.

Mr. Warner: You said that the waves first issuing from the source of light have the greater amplitude. Have they any greater length?

Prof. Morley: No increase in length has been detected. The first of a series of luminous vibrations emanating from a disturbed atom have a greater amplitude, just as the first vibrations from a musical instrument of percussion have the greater amplitude.

As spherical waves pass out from their source, they have work to do on a larger scale, therefore their amplitude decreases. Imagine that an atom of sodium in a flame suffers a collision which sets it in motion. It continues to vibrate, and like a piano string, with lessening amplitude; these vibrations are propagated outwards in all directions. As they pass outwards their amplitude decreases. Now a sodium atom may