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APPENDIX.
(From Biot's Additions to Fischer's Physique Mechanique.)
Many optical phenomena relating to the physical properties of light, having of late years acquired some importance, we will here give, not a detailed account of them, which would not suit the plan of this Work, but a sketch which will indicate the principal results.
Coloured Rings.
When two plates of glass whose surfaces are not quite plane, are placed one on the other, the lamina of air naturally adhering to those surfaces, has usually thickness enough to exercise complete action on light, that is, it reflects and refracts all the coloured rays in the same manner as if it were of considerable depth. If, however, one of the glasses be rubbed on the other, and forcibly pressed to it, to exclude a part of the intermediate air, there will soon be perceived a degeee of adhesion, which is generally greater in some parts than in others, either because the surfaces are always a little curved, or because they invariably bend under strong pressure; in this manner there is obtained a lamina of air, thinner than the preceding, and the depth of which increases gradually in all directions from the point in which the surfaces are most closely in contact. If now these glasses be turned so that the eye may receive the light of the clouds, reflected by the lamina of air, there will be perceived a number of concentric coloured rings, which, when the glasses are pressed sufficiently, surround a dark spot, at the point of contact.
These coloured rings may be formed by pressing together transparent plates of any other substance, besides glass; they may be observed, when a glass lens is placed on a plane surface of resin, of metal, of metallic glass, or any other polished body. These rings subsist moreover in the most perfect vacuum that can be produced. Neither is it necessary for their formation, that the interposed lamina be of air, nor that it be contained between two solid substances: a
