721.
To place the doctrine of colours nearer, in this sense, within the philosopher's reach, was the author's wish; and although the execution of his purpose, from various causes, does not correspond with his intention, he will still keep this object in view in an intended recapitulation, as well as in the polemical and historical portions of his work; for he will have to return to the consideration of this point hereafter, on an occasion where it will be necessary to speak with less reserve.
RELATION TO MATHEMATICS.
722.
It may be expected that the investigator of nature, who proposes to treat the science of natural philosophy in its entire range, should be a mathematician. In the middle ages, mathematics was the chief organ by means of which men hoped to master the secrets of nature, and even now, geometry in certain departments of physics, is justly considered of first importance.
723.
The author can boast of no attainments of this kind, and on this account confines himself to departments of science which are independent of geometry; departments which in modern times have been opened up far and wide.