The two lapped surfaces whose separation defines the length of a good end-gauge or block-gauge generally approach a degree of optical flatness and parallelism sufficient for their use in interferometry. If, therefore, such a gauge is supported between the semi-transparent mirrors of a Fabry-Perot étalon of greater length than the gauge, with its surfaces parallel to the mirrors, the gauge may be standardised in terms of light waves by the methods usually applied to Fabry-Perot étalons. In fig. 1 L is the optical separation of the étalon mirrors, l 1 and l 2 are respectively the optical separations of a gauge surface and an adjacent étalon mirror at each end of the figure, and O is the optical length of the gauge: therefore O = L — ( l 1 + l 2 ), (1) L is measured either directly or indirectly in terms of light waves, the choice depending upon the magnitude of L, while both l 1 and l 2 are measured directly in terms of light waves by observation of the reflected system of circular interference fringes; thus O may be obtained in terms of light waves.

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It is well known that two clean glass plates with optically worked flat surfaces adhere to one another when brought into intimate contact. Considerable force is required to separate the plates; in fact, it often happens that their surfaces become damaged during the process. Such adherence is clearly due to molecular cohesion. Similar evidence of cohesion is observed when contact is made between accurately flat, polished surfaces of steel and of steel and glass.

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