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That there must be a medium forming a continuous material communication throughout space to the remotest visible body is a fundamental assumption in the undulatory Theory of Light. Whether or not this medium is (as appears to me most probable) a continuation of our own atmosphere, i ts existence is a fact t ha t cannot be questioned, when the overwhelming evidence in favour of the undulatory theory is considered; and the investigation of its properties in every possible way becomes an object of the greatest interest. A first question would natural ly occur, What is the absolute density of the luminiferous ether in any par t of space ? I am not aware of any a t tempt having hitherto been made to answer this question, and the present state of science does not in fact afford sufficient data. I t has, however, occurred to me t h a t we m a y assign an inferior limit to the density of the luminiferous medium in interplanetary space by considering the mechanical value of sunlight as deduced in preceding communications to the Royal Society from POUILLET'S data on solar radiation, and JOULE'S mechanical equiva^ lent of the thermal unit . Thus the value of solar radiation per second per square foot a t the earth 's distance from the sun, estimated a t *06 of a thermal uni t centigrade, or 83 foot-pounds, is the same as the mechanical value of sunlight in the luminiferous medium through a space of as ma n y cubic feet as the number of linear feet of propagation of light per second. Hence the mechanical value of the whole energy, actual and potential, of the disturbance kept up in t h e space

VI.—Note on the Possible Density of the Luminiferous Medium and on the Mechanical Value of a Cubic Mile of Sunlight