to prove that mathematical knowledge is
essential in theology, and closes this section of his work with two
comprehensive sketches of geography and astronomy. That on geography is
particularly good, and is interesting as having been read by Columbus, who
lighted on it in Petrus de Alliaco's _Imago Mundi_, and was strongly
influenced by its reasoning.

Part V. (pp. 256-357) treats of perspective. This was the part of his work
on which Bacon most prided himself, and in it, we may add, he seems to owe
most to the Arab writers Kindi and Alhazen. The treatise opens with an able
sketch of psychology, founded upon, but in some important respects varying
from, Aristotle's _De Anima_. The anatomy of the eye is next described;
this is done well and evidently at first hand, though the functions of the
parts are not given with complete accuracy. Many other points of
physiological optics are touched on, in general erroneously. Bacon then
discusses vision in a right line, the laws of reflection and refraction,
and the construction of mirrors and lenses. In this part of the work, as in
the preceding, his reasoning depends essentially upon his peculiar view of
natural agents and their activities. His fundamental physical maxims are
matter and force; the latter he calls _virtus_, _species_, _imago agentis_,
and by numberless other names. Change, or any natural phenomenon, is
produced by the impression of a virtus or species on matter--the result
being the thing known. Physical action is, therefore, _impression_, or
transmission of force in lines, and must accordingly be explained
geometrically. This view of nature Bacon considered fundamental, and it
lies, indeed, at the root of his whole philosophy. To the short notices of
it given in the 4th and 5th parts of the _Opus Majus_, he subjoined two, or
perhaps three, extended accounts of it. We possess at least one of these in
the tract _De Multiplicatione Specierum_, printed as part of the _Opus
Majus_ by Jebb (pp. 358-444). We cannot do more than refer to Charles for
discussions as to how this theory of nature is connected with the
metaphysical problems of force and matter, with the logical doctrine of
universals, and in general with Bacon's theory of knowledge.

Part VI. (pp. 445-477) treats of experimental science, _domina omnium
scientiarum_. There are two methods of knowledge: the one by argument, the
other by experience. Mere argument is never sufficient; it may decide a
question, bu