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Two years before the publication of his Dioptrics, viz. in 1609, Kepler had given to the world his great work, entitled "The New Astronomy, or Commentaries on the Motions of Mars." The discoveries which this volume records form the basis of physical astronomy. The inquiries by which he was led to them began in that memorable year 1601, when he became the colleague or assistant of Tycho.
Though artisans may not be able at once to execute the invention which is explained in the "Dioptrics," I do not think that any one on that account is entitled to condemn it; for since address and practice are required in order so to make and adjust the machines described by me as not to overlook the smallest particular, I should not be less astonished if they succeeded on the first attempt than if a person were in one day to become an accomplished performer on the guitar, by merely having excellent sheets of music set up before him.
Kepler's nominal salary should have been ample for his expenses, increased though they were by his growing family, but in the depleted state of the treasury there were many who objected to any payment for such "unpractical" purposes. In 1604 he published "A Supplement to Vitellion," containing the earliest known reasonable theory of optics, and especially of dioptrics or vision through lenses.
In the field of optics, also, Huygens has added considerably to science, and his work, Dioptrics, is said to have been a favorite book with Newton. During the later part of his life, however, Huygens again devoted himself to inventing and constructing telescopes, grinding the lenses, and devising, if not actually making, the frame for holding them.
Just so; but do not you see that as soon as the mind has got any ideas at all, every judgment is a process of reasoning? So that as soon as we compare one sensation with another, we are beginning to reason. The art of judging and the art of reasoning are one and the same. Emile will never learn dioptrics unless he learns with this stick.
The only other optical treatise written by Kepler, was his Dioptrics, with an appendix on the use of optics in philosophy. This admirable work, which laid the foundation of the science, was published at Augsburg in 1611, and reprinted at London in 1653.
If some of the matters of which I have spoken in the beginning of the "Dioptrics" and "Meteorics" should offend at first sight, because I call them hypotheses and seem indifferent about giving proof of them, I request a patient and attentive reading of the whole, from which I hope those hesitating will derive satisfaction; for it appears to me that the reasonings are so mutually connected in these treatises, that, as the last are demonstrated by the first which are their causes, the first are in their turn demonstrated by the last which are their effects.
Kepler's Pecuniary Embarrassments His Inquiries respecting the Law of Refraction His Supplement to Vitellio His Researches on Vision His Treatise on Dioptrics His Commentaries on Mars He discovers that the orbit of Mars is an Ellipse, with the Sun in one focus And extends this discovery to all the other Planets He establishes the two first laws of Physical Astronomy His Family Distresses Death of his Wife He is appointed Professor of Mathematics at Linz His Method of Choosing a Second Wife Her Character, as given by Himself Origin of his Treatise on Gauging He goes to Ratisbon to give his Opinion to the Diet on the change of Style He refuses the Mathematical Chair at Bologna.
Besides the "Discourse on Method" , with the treatises on dioptrics, meteors, and geometry, his principal works were his "Meditations" addressed to the Deans of the Faculty of Theology in the University of Paris; the "Principia Philosophiae," and the "Traité des Passions de L'Ame," in which, he handled morals.
And when the lovers of the art of painting are so enamoured with the charmingness of their own performances, that Nicias, as he was drawing the Evocation of Ghosts in Homer, often asked his servants whether he had dined or no, and when King Ptolemy had sent him threescore talents for his piece, after it was finished, he neither would accept the money nor part with his work; what and how great satisfactions may we then suppose to have been reaped from geometry and astronomy by Euclid when he wrote his Dioptrics, by Philippus when he had perfected his demonstration of the figure of the moon, by Archimedes when with the help of a certain angle he had found the sun's diameter to make the same part of the largest circle that that angle made of four right angles, and by Apollonius and Aristarchus who were the inventors of some other things of the like nature?
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