The following Lectures were delivered at the Society of Arts in the year 1869. They have been carefully revised, and in some parts expanded.<br><br>I have to thank Professor Roscoe for permission to use several of the woodcuts which appear in his "Spectrum Analysis," to which admirable work the present small volume will, I trust, be found useful as an introduction.<br><br>My acknowledgments are also due to Professor Pedler, both for aid in the experimental illustrations when the Lectures were delivered, and for his assistance while the revision has been passing through the press.