AUTHORS P R E F A C E - THE importance so long assigned to the property, possessed by many orgauic substances, of rotating the plane of polarization, alike in its theoretical and pFactical aspects, rnakes it rather surprisilig that hitherto the only help available f thre study of the subject was to be found in the various mernoirs, in which the necessary information lay scattered, in scientific journals. The present work was undertaken with a view to relieve the inconvenience arising from the want of a comprehensive treatise. It is based upon a paper of mine published some time ago in Liebiyas Annalen, Bd. 189, in which I discussed the mode of determining specific rotation, and, by way of introduction, gave a brief general account of optical activity. Since then requests have frequently reached me, urging the desirability of extending that paper by including an account bf all the recent instruments and the practical applicatiocs, so as to make it a complete monograph of the subject. I was the more readily induced to undertake the task by the fact that, of late years, the increased attention given to thi ihenomena of rotation has brought to light such a rnass of facts as makes it possible to present the material in a more or less complete form. In its theoretical aspect the optical activity of organic substances possesses high interest. As it is a consequence of peculiarity of arrangement of the atoms in the molecule, it must assuredly afford some assistance in determining the constitutiona, l formula to be assigned to the substances. The investigation of this connection between optical power and chemical constitution is, indeed, of the utmost importance, and givea promise of a rich harvest for future workers. For this purpose it is, above all, necessary that determinations of specific rotation should be made with the most rigid accuracy. Special attention has therefore been paid in the present work to the needs of the scientific investigator, by affording a detailed account of the different polarimet. ric instruments, and the methods of observation of specific rotation, as well as of other data connected therewith. The methods described are those which ensure the highest accuracy, and care has been taken to indicate in each case the limits of accuracy attainable. Where only a rougher estimate is required, it will readily be seen that steps may be omitted so as to simplify the proceas. The importance of the subject, from a practical point of view, has been long acknowledged in its application. to the determination of sugar, and recently of other substances, more especially the cinchona bases. The methods of observation in these special cases have been fully treated, and the sugar-chemist in particular will find interest and novelty in the account of the different saccharirneters, and the corrections to be applied to the result. The introductory chapter on the optical principles of the subject may, perhaps, be not unwelcome to many a chemist. It has been made as elementary and succinct as possible. The relation between rotatory power and crystalline form, as belonging rather to crystallographic physics, has only been briefly touched upon. For any further account of the work the reader need only be referred to the table of contents, which has been made as coniplete as possible. HANS LANDOLT. TRANSLATORS YREFACE. A FEW words will suffice to introduce the present edition to the English reader. Some months ago, Mr. Frank Faulkner, the energetic and intelligent brewer of St. Helens and Beeston, to whom the English public are alreadylindebted for the appearance of Pasteurs Studies on Fermentation, placed in my hands, for revision and editing, a manuscript translation1 of Dr...