. Carnegie Institution of Washington publication. 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 her
. Carnegie Institution of Washington publication. 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 micrometers, so that. 58 THE INTERFEROMETRY OF corresponding displacements e, e, normal to G and G and y in the directionbb, may be registered. The adjustments,if symmetry were demanded, would be cumbersome; for,in addition to precise modification of the position and orientation of the prismsP, P, the grating requires fine adjustment and a means of securing parallelismof the rulings. But an approximate adjustment does very well and no painswere taken in the first experiments to secure symmetry. The spectra wereintensely brilliant in the low-order work; but even in the fourth order thelight was adequate. One may note that here the gratings do not reverse thedispersion of the prism P, though this is relatively small. Table 12 is anexample of results: TABLE 12.—Ranges gratings and of displacement varying with dispersion. Paired60° prism. ^ = 46°. 8=44°. x — 2ecosS/2. Order ObservedeXio3 rcXio3 i dd/d\ Remarks. cm. cm. 0 38 70 22° 760 i 1 802001 80 35. ) ° J 31-2° } 3,490 200 351 a 4204
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