THE importance so long assigned to the property, possessed by many
organic substances, of rotating the plane of polarization, alike in its
theoretical and practical aspects, makes it rather surprising that
hitherto the only help available for the study of the subject was to be
found in the various memoirs, 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 Liebig's Annalen, Bd. 189, in which I discussed
the mode of determining specific rotation, and, by way of introduc-
tion, 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 of all the recent
instruments and the practical applications, so as to make it a complete
monograph of the subject. I was the more readily induced to under-
take the task by the fact that, of late years, the increased attention
given to the phenomena of rotation has brought to light such a
mass 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 arrange-
ment of the atoms in the molecule, it must assuredly afford some
assistance in determining the constitutional 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 gives promise of a rich harvest for future workers.