The subject of the following Memoir has been already touched on in a communication which I made to the Cambridge Philosophical Society, and which appears in the seventh volume of their Transactions, Art. IX. For the better understanding of what follows, it is desirable to state briefly the contents of that paper. Its object is to estimate the quantity of light which is received on a screen of unlimited dimensions, after passing through a certain aperture, or suffering reflection at two mirrors, as in the case of the interference experiment, so as to exhibit the appearance of bands.