Michelson A. A. Light waves and their uses (1903)

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Microscope, Telescope, Interferometer 31

100,000 to the inch. This is about the theoretical limit for blue light. By using the portion of the spectrum beyond the violet it might be possible to go still farther.

Doubtless by a much higher magnification a much more accurate setting on a given phase of the fringes could be made, and hence a corresponding increase of accuracy of measurement could be attained. But this involves a great loss of light, since the intensity varies inversely as the square of the magnification. Consequently, even with a threefold magnification the intensity is diminished ninefold, so that it would be difficult to see the image unless the illumi

32 Light Waves and Their Uses

nation we*re so powerful as to endanger the specimen, or to introduce* temperature variations which would vitiate the results of the measurement.

It is apparent from all that precedes that in all measurements by the microscope or the telescope we are, in fact,

FIG. 26

making use of the interference of light waves. Let us see, then, if we are making the best use of this interference*, or whether it may not 1 Mi possible to increase the high degree of accuracy already attained.

It has just, been shown that, in the case of a telescope, tin* angular magnitude* of the diffraction rings, and with this the* accuracy of me»asure*ment. e>f the* position of the luminous point, depends only on the* diameteT of the* objective*. Now. the form of the* fringe*s will of course vary with the* form of the aperture*, and if this be* sepiare* instead of circular, the* diffraction image* will be* represente*d bv Fig. 2(*>, which may be compared with Fig. 23. The* width of the* fringes is but littl«‘ altered, while* then* is a perceptible increase in dis