. Carnegie Institution of Washington publication. fracting glass. In contrast with the ease in which the adjustments weremade with the symmetrical 45° prisms, the corresponding work with 60° prismsproved to be exceedingly difficult. True, the prisms were large, includinglong glass-paths. The sodium lines, now clearly separated, were markedlycurved, so that on placing them near together they either assumed an O-shapeor an X-shape. But the spectra were brilliant and nothing appeared to mili-tate against a successful result. But it was not until after days of searchingthat the fringes were found,
. Carnegie Institution of Washington publication. fracting glass. In contrast with the ease in which the adjustments weremade with the symmetrical 45° prisms, the corresponding work with 60° prismsproved to be exceedingly difficult. True, the prisms were large, includinglong glass-paths. The sodium lines, now clearly separated, were markedlycurved, so that on placing them near together they either assumed an O-shapeor an X-shape. But the spectra were brilliant and nothing appeared to mili-tate against a successful result. But it was not until after days of searchingthat the fringes were found, and then only with two prisms of different, highlyrefracting glass. They were not quite uniform, but it seemed impossible toimprove them. The ranges of displacement were found to be to with the electric arc, cm. with (condensed) sunlight. This is againin accord with the large increase of dispersion, the range of displacement beingabout five times greater than was the case with 45° prism of less refractingglass. sf> 37. 25. Diffraction at M, N, replacing reflection.—The present method ofobserving interferences in the zero, first, second, third, and even fourthorder, successively, without essential change of the parts of the apparatus, isnoteworthy. I happened to possess a plane reflecting grating (D X lo6 = 200),cut into two equal parts by a section parallel to the rulings, and it was there-fore easy to devise the method. In figure38, the incident light L from the colli-mator is separated into two componentbeams a and a by the 60° prism P. Thisis essential here, as an abundance of lightis needed (sunlight should be focused bya large lens of long focus (5 feet) on theslit). The rays a, a are then eitherreflected or diffracted in any order by theplane reflecting gratings G, G into the collinear rays b, b. They are thenreflected by the silvered right-angled prism P and observed in a telescopeat T. G and G and if possible also P should be on micrometer
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