. Carnegie Institution of Washington publication. THE AID OF THE ACHROMATIC FRINGES. 75. •as 15 cm. long and cm. in diameter, but to be changed in capacity with the balls m,m' used. The stems of the hooks, screwed and sealed into the tube rr, carry the mirrors n, n'. The needle is suspended from a torsion-head by the bifilar of silk fiber kl, also com- pletely submerged in the water-bath. The hooks k and I are provided with a thin flat sheet-metal link, by which the bifilar may be appropriately spaced. The needle system is so adjusted as just to float. It is then weighted by i or 2 grams t
. Carnegie Institution of Washington publication. THE AID OF THE ACHROMATIC FRINGES. 75. •as 15 cm. long and cm. in diameter, but to be changed in capacity with the balls m,m' used. The stems of the hooks, screwed and sealed into the tube rr, carry the mirrors n, n'. The needle is suspended from a torsion-head by the bifilar of silk fiber kl, also com- pletely submerged in the water-bath. The hooks k and I are provided with a thin flat sheet-metal link, by which the bifilar may be appropriately spaced. The needle system is so adjusted as just to float. It is then weighted by i or 2 grams to sink it. The weight in water may be measured at K. In view of the weight of needle, the mirrors n,n' may be rigidly connected by a very nar- row strip of this plate glass, which facili- tates adjustment. The attracting weights M and M' up to 5 kg. were used in pairs M,Mi and M'.M'i, on each side suspended by a steel belt from a pulley overhead in such a way that when MM' are in Place M'iMi are raised out of effective reach. The chief difficulty was encountered in floating the needle. When this was done the whole tank was fastened in place on the interferometer, as the torsion-head at k is attached adjustably to the tank. The fringes were found without much difficulty; but they were in incessant motion, owing no doubt to eddy currents produced by temperature differ- ences. After many trials I concluded that measurements would be untrust- worthy and further trials were therefore abandoned. The experiment is in fact too difficult for a single observer and would be feasible only in an environment of perfect quiet and constant temperature. 43. Expeditious fringe detection.—In work like the present it is necessary to find the fringes quickly under considerable disadjustment of parts. In this case, if the auxiliary mirror m (fig. 4, Chapter I) can be manipulated, the two images in the telescope T are first made coincident by adjusting M', in which case the rays are parallel as t
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