Optics

...I have set about the writing. Considering also that the same power of form is sent into the opposite medium and into the sense, and that light is the first of all sensible forms, and that we intend to seek out the efficient causes of all sensible things, of which sight reveals many differences to us. The investigation of the aforementioned things by way of visible beings was pleasing, just as it was to many men who treated the business of this science before us, calling them by the name of PERSPECTIVE; whose naming I also approve of as pleasing: although the intention of the writer inclines more toward treating the most hidden mode of the action of natural form, so that the present work may correspond to your desires. For that which is acted upon more perceptibly in the sense of sight is in no way avoided in natural things in the absence of the sense itself. For the presence of the sense adds nothing to the actions of natural forms. Therefore, running through every mode of vision by mathematical or natural demonstration, I shall treat those things concerning the natural actions of forms by way of visible affections according to a threefold mode of seeing, according to the measure of my ability. For in all those modes of seeing, natural forms diffuse themselves to the sight, and visual rays do not go out to seize the forms of things. Whence if sight were not present to the presence of forms diffused through natural bodies capable of receiving them, there will not on this account be no natural action, but the forms will impress themselves as much as they can upon the subject bodies dissimilar to themselves. You, therefore, man of desires for every scientific good, receive what you commanded to be done; in which, if you find anything unpolished, may you refine it with your more perspicacious intellect.

THE DIVISION OF THE WHOLE WORK INTO TEN BOOKS, and what is treated in each.

Decorative initial P containing leafy patterns and scrollwork within a rectangular frame.

WE have therefore thought it best to distinguish the present business into ten partial books. For wishing to conclude every visible being, as an affection happens to its visibility, by mathematical demonstration, and to walk more surely in this way as far as it is possible for us, we have made this book self-standing, except for those things which depend on the Elements of Euclid, and a few things which depend on the Conic Elements of Apollonius of Perga, which are only two propositions which we have used in this science, as will appear later in the process. In the first book of this science, therefore, we set forth axioms which are necessary for this science beyond the Elements of Euclid. And in this, we explain those two things which were demonstrated by Apollonius. Many of those things which we set forth in this book are also contained in that book which we call On Elementated Conclusions, in which we have written down universally everything that seemed true to us, and which reached us from men later than Euclid, universally concluded for the necessity of particular sciences. In this, our second book, we have also treated the mode of projection of rays through the medium of one transparent body, or several, upon diverse figures of bodies; likewise, concerning the projection of shadows and the figuration of light falling through windows, as being among those things that are preambles to the sensible action of natural forms, and which occur even when the sense does not exist. In the third book, indeed, we have treated the organ of sight and the essential mode of seeing in its own way, as the science of Optics allows. In the fourth book, also, we run through the deceptions which happen to sight according to the direct mode of seeing through one medium, whether they be mathematical or even natural affections. In the fifth book, however, transferring ourselves to another mode of seeing, which happens through reflections from polished bodies, which we call mirrors, we have treated the affections common to every mirror, whether it be plane, spherical, columnar or pyramidal, concave or convex. For these are all the mirrors from which a regular reflection can occur, as we shall explain in its place; nor, however, do we understand by these mirrors only bodies polished by art, but rather by nature. Because while we apply the demonstration to these mirrors, we understand natural bodies of the same figure. For what happens irregularly in artificial bodies must necessarily happen more certainly in natural bodies. And while we thus run through the figures of mirrors, celestial and all...