publishers of the magazines from which these essays have been taken, for permission to reproduce them. Some readers may, perhaps, be disposed to ask whether the present volume and my Essays on Astronomy' complete my treatment of the subjects discussed in them. I must frankly admit that they do not. There is no finality in science; and not only must each new discovery respecting the celestial orbs suggest new ideas, but every re-examination of the information already obtained must throw a fresh light on problems of interest connected with them. I cannot promise my readers, then, that at some future time I may not have additional matter to communicate about the sun and moon, planets and meteors, stars, comets, and nebulæ. But I can promise that if, encouraged by the kindness thus far extended to me, I continue to deal with subjects about which I have already written much, it will be only that I may discuss new discoveries, adduce fresh arguments, and describe original researches. RICHARD A. PROCTOR. BRIGHTON: May, 1872. B THE ORBS AROUND US. INTRODUCTION. THE GAMUT OF LIGHT. A YEAR OR TWO AGO the writer of the present paper had occasion, during the course of a lecture on astronomy, to explain the nature of spectroscopic analysis to a mixed audience. He had gone through the usual statement of the laws on which this mode of research depends; but he felt convinced that the explanation had been insufficient. That sense of sympathy which enables every lecturer to tell whether his hearers are following him, assured the writer that his audience, with every readiness to be convinced, had not grasped the essential principle on which spectroscopic analysis depends. It will be understood that his object was not to give a complete account of the new analysis; but it was essential for his purpose that the convincing nature of the evidence which the analysis affords should be brought clearly before his audience. He knew that B |