The Bakerian Lecture. Experiments and calculations relative to physical optics

Abstract

It consists of six sections, the first of which is intended to convey an experimental demonstration of the general law of the interference of light. This demonstration rests on two experiments, the results of which are brought in proof, that fringes of colours, and even the crested fringes described by Grimaldi, are produced by the interference of two portions of light. These results are, that if one of the two edges of a shadow produced by a narrow opake body be intercepted by a screen at a small distance from that body, the opposite edge will no longer exhibit the fringed appearance which it had in common with the former edge, when the latter was not intercepted. Under the second head we have a comparison of measures of the intervals of disappearance of light when refracted between two edges of knives, or intercepted by a hair or a thin wire. The experiments, which were partly suggested by some observations of Sir Isaac Newton are here collected in tables: and the author states, as a general inference, that if we thus examine the dimensions of the fringes under different circumstances, we may calculate the differences of the lengths of the paths of the portions of light which have been proved to be concerned in producing those fringes; and we shall find, that where the lengths are equal, the light always remains white; but that where either the brightest light, or the light of any given colour, disappears and reappears a first, a second, or a third time, the differences of the paths of the two portions are nearly in an arithmetical progression.