. Carnegie Institution of Washington publication. 4 The values of dd/dNc observed and computed obviously agree as closelyas may be expected for mean conditions of the spectrum between green andred and of the mode of procedure. The small coefficient for carbon bisul-phide is, moreover, in keeping with its large value of B or high a sensitive displacement interferometer B like E D should all be small;but unfortunately this implies large ellipses. The beautiful small ellipsesobtained with the carbon-bisulphide plate are of little avail because of theirsluggish motion. In fact, the s
. Carnegie Institution of Washington publication. 4 The values of dd/dNc observed and computed obviously agree as closelyas may be expected for mean conditions of the spectrum between green andred and of the mode of procedure. The small coefficient for carbon bisul-phide is, moreover, in keeping with its large value of B or high a sensitive displacement interferometer B like E D should all be small;but unfortunately this implies large ellipses. The beautiful small ellipsesobtained with the carbon-bisulphide plate are of little avail because of theirsluggish motion. In fact, the small thickness of the carbon bisulphide layerand the reduced coefficient observed are quite striking in comparison with ^lass. Moreover, since for an order n o dd i X2 dn 2 D cos <J (cos R + zB/\z cos R) — N the effect of a large B, though it reduces the general size of ellipses, does sobut slightly, since the term in B can not be more than a few per cent (3 to 6)of the other term in the binomial. CHAPTER THE DISPLACEMENT INTERFEROMETRY OF SMALL ANGLES AND OF LONGDISTANCES. COMPLEMENTARY FRINGES. 67. Parallel rays retracing their path.—The following method was devisedwith a view to the micrometric measurement of angles. It will be used else-where in connection with an electrometer for reading microvolts. An inter-ference method of a different kind for measuring small angles was developedsome time since and used at length in connection with the deviation of thehorizontal pendulum.* Again, theelectrometer was treated in differentwayst by the aid of the interferom-eter. The present method, however,will differ from all of these. In figure88, L is a horizontal beam of whitelight from a collimator after passingthrough the auxiliary clear plate P(to be used preliminarily for paral-lelizing the mirrors of the systemin a way presently to be shown),the beam is reflected at a and 6 by the half-silver plates H \ and Hi respect-ively, to the wide opaque mirror in. The rays now r
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