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.

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