A study of glass surfaces in optical contact

Abstract

It is known that two very flat glass or silica surfaces can by suitable manipulation be brought into "optical contact", contact so close that the reflexion at the interface, if any, is very small. The most important practical application of this principle lies in certain cases of optical construction initiated by Messrs. Adam Hilger, Ltd. The plates of transmission echelons are "contacted" in this way to avoid loss of light by reflexion. The same applies to the right- and left-handed halves of Cornu quartz prisms, and to Lummer cubes used in photometry. More recently, Mr. E. J. Williams has applied the principle to the construction of his reflexion echelons. In this latter case the advantage lies not in transparency but in the accuracy with which the parts can be put into their proper mutual relations. The distance apart of the "contacted" surfaces is definite to within a small fraction of a wave-length. At the same time they are held together by considerable force, so that their relative position is well secured, even without clamping. Little seems to have been done in the way of studying this optical contacting process for the sake of its own inherent interest, apart from the application. The present paper is a step in this direction.