Preface to the Third Edition
xii
first or second edition. Undoubtedly, the natural thing was to select
the later version for this purpose. How could an editor choose to dis¬
regard any alterations or additions which had been made by the author
himself? If, therefore, in spite of so obvious an objection, the editors
have preferred the text of the first edition, there must have been special
reasons that were responsible for this decision. Foremost of these was
the consideration that the whole imperishable significance of Helm¬
holtz’s achievements in the domain of the physiology of vision, the
elegant physical methods which he adopted and improved, his pains¬
taking observations of the sensations themselves and the allied
psychical phenomena, the mathematical analysis and philosophical
and critical discussion, all these characteristic features are essentially
interwoven with the first edition of the Physiological Optics. It is the
classical work that marked the dawn of a new era in the science of the
physiology of the senses.
However, there was another reason also for preferring the text of
the original edition. Curiously enough the changes and additions
which were introduced in the second edition seem to be called in
question and discarded at present to a far greater extent than the
contents of the first edition.* The explanation of this is hardly to be
attributed to the fact that at the time when Helmholtz was at work
on the second edition, he was diverted from the task by other absorbing
labours and consequently could not give to physiological optics the
same undivided strength and interest as formerly, and so had not been
able to keep pace with the more recent developments in this domain
of science. A sufficient answer to that is that at this very time, under
his influence and with his collaboration, A. König’s important re¬
searches were begun and had aroused his keenest interest. Through
these investigations he had become acquainted with part of a newly
explored region. Subsequent study of these phenomena and better
knowledge of them placed the results in a somewhat different light
and suggested the probability of a different interpretation. It is easy
for us looking backwards to see now how, on account of the unfinished
state of these researches, the beginning of the year 1890 happened to
be a singularly unfortunate time for a new edition of the Physiological
Optics. Thus at the present time it is not only easier but wiser to
undertake the revision on the safe basis of the first edition instead of
attempting to reconcile the tentative investigations in the second
edition. This criticism applies, of course, to only one part of physi¬
ological optics; but it is the very part in which the changes in the
second edition are most numerous.
* It will be of interest to read the review of the second edition written by Professor
J. McKeen Cattell and published in Science, N.S. Vol. VIII, pp. 794-796. Dec. 2, 1898.
(J. P. C.S.)