. The London, Edinburgh and Dublin philosophical magazine and journal of science . (fig. 1), a sodium-flame, falls onthe plane parallel glass a, and is divided, part going to theplane mirror c, and part to the plane mirror b. These twopencils are returned along cae and bae, and the interferenceof the two is observed in the telescope at e. If the distancesac and ah are made equal, the plane c made parallel with thatof the image of b, and the compensating glass d interposed,the interference is at once seen. If the adjustment be exact,the whole field will be dark, since one pencil experiencesexte
. The London, Edinburgh and Dublin philosophical magazine and journal of science . (fig. 1), a sodium-flame, falls onthe plane parallel glass a, and is divided, part going to theplane mirror c, and part to the plane mirror b. These twopencils are returned along cae and bae, and the interferenceof the two is observed in the telescope at e. If the distancesac and ah are made equal, the plane c made parallel with thatof the image of b, and the compensating glass d interposed,the interference is at once seen. If the adjustment be exact,the whole field will be dark, since one pencil experiencesexternal reflexion, and the other internal. If now b be moved parallel with itself a measured distanceby means of the micrometer-screw, the number of alternationsof light and darkness is exactly twice the number of wave-lengths in the measured distance ; thus the determinationconsists absolutely of a measurement of a length and thecounting of a number. The degree of accuracy depends on the number of wave-lengths which it is possible to count. Fizeau was unable to Figs. 1 and 2. u m. \S observe interference when the difference of path amounted to50,000 wave-lengths. It seemed probable that with a smallerdensity of sodium vapour this number might be increased, and of Sodium as the Standard of Length. 465 the experiment was tried with metallic sodium in an exhaustedtube provided with aluminium electrodes. It was found pos-sible to increase this number to more than 200,000. Now itis very easy to estimate tenths or even twentieths of a wave-length, which implies that it is possible to find the number ofwave-lengths in a given fixed distance between two planes withan error less than one part in two millions and probably onein ten millions. But the distance corresponding to 400,000wave-lengths is roughly a decimetre, and this cannot be deter-mined or reproduced more accurately than, say, to one part in500,000. So it would be necessary to increase this can be done by using th
Size: 2710px × 922px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1840, bookidlondon, booksubjectscience
