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<p>Interference bands are produced between two optically true glass surfaces in the usual way. Any small movement of one surface relative to the other can be readily seen by the changes in the bands. Several instruments are described; the agents used to change the position of the surfaces, being (1) pressure of a gas, /b e.g./, the pressure produced in a stethoscope by the beating of an artery and communicated to the interference apparatus by a rubber tube; (2) heat produced by an electric current passing along an isthmus of thin metal between the optical surfaces; (3) electromagnetic movement of a telephone disphragm due to a courrent; (4) electrostatic attraction between two metal films when the p.d. between them is varied. Rapid alternations would n
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<br>Subjects: interference</div>
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