. Carnegie Institution of Washington publication. owever, in any order, as was the casewith the Jamin interferometer, although many trials with different compen-sators were made. They would not be useful for measurement. Removingwhite light and replacing it by sodium light, the white fringes were found tobe of exceedingly high order, more than cm. of micrometer-screw beingneeded before the circles of the yellow field were approached. The latterwere very vague and usually not seen in the principal focal plane of the tele-scope. Since the rays retrace their path in the Michelson interferomet
. Carnegie Institution of Washington publication. owever, in any order, as was the casewith the Jamin interferometer, although many trials with different compen-sators were made. They would not be useful for measurement. Removingwhite light and replacing it by sodium light, the white fringes were found tobe of exceedingly high order, more than cm. of micrometer-screw beingneeded before the circles of the yellow field were approached. The latterwere very vague and usually not seen in the principal focal plane of the tele-scope. Since the rays retrace their path in the Michelson interferometer, theraising and lowering of the spectrum ellipses is not possible; but the residualfringes may be put in any order by changing the differential glass-path of therays. They appear but once, not rhythmically like the sodium fringes. The equation for this phenomenon would thus be (since the rays retracetheir paths) n\ = 2eiJ, cos (r— a) where e is the thickness of the single half -silver and a its effective wedge angle,positive or negative. 85. 84 To obtain the circular fringes in Michelsons interferometer with a wideslit and homogeneous light, the rays must rigorously retrace their path—, all reflection must take place at the same spot on the half-silver this is the case the fringes, even though obtained with common plate-glass and a sodium flame, are beautifully circular and sharp. This is due tothe fact that so small a part of the plate is used. They are stationary andexhibit the Fizean periods due to the doublet DJDi, admirably. With whitelight the fringes are faint and useless. When the rays do not accuratelyretrace their paths—, when there are two spots of light on the half-silverone or more centimeters apart—the fringes are soon linear and very small,as above. With regard to the last equation, if N is the difference of normal distancesto the two opaque mirrors (M, N) of the Michelson interferometer, from the REVERSED AND NON-REVERSED SPECTRA. 1
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