P. KELLAND, M.A., F.R.S., FORMERLY FELLOW OF QUEENS' COLLEGE, CAMBRIDGE; AND P. G. TAIT, M.A., FORMERLY FELLOW OF ST PETER'S COLLEGE, CAMBRIDGE; PROFESSORS IN THE DEPARTMENT OF MATHEMATICS IN THE PREFACE. THE present Treatise is, as the title-page indicates, the joint production of Prof. Tait and myself. The preface I write in the first person, as this enables me to offer some personal explanations. For many years past I have been accustomed, no doubt very imperfectly, to introduce to my class the subject of Quaternions as part of elementary Algebra, more with the view of establishing principles than of applying processes. Experience has taught me that to induce a student to think for himself there is nothing so effectual as to lay before him the different stages of the development of a science in something like the historical order. And justice alike to the student and the subject forbade that I should stop short at that point where, more simply and more effectually than at any other, the intimate connexion between principles and processes is made manifest. Moreover in lecturing on the groundwork on which the mathematical sciences are based, I could not but bring before my class the names of great men who spoke in other tongues and belonged to other nationalities than their own-Diophantus, Des Cartes, Lagrange, for instance-and it was not just to omit the name of one as |