Introduction to the Literature of Europe in the Fifteenth, Sixteenth, and Seventeenth Centuries, Vol. 2

by Henry Hallam

[637] Vol. vii., p. 437.

Optics.—Discoveries of Kepler. 45. The science of optics was so far from falling behind other branches of physics in this period, that, including the two great practical discoveries which illustrate it, no former or later generation has witnessed such an advance. Kepler began, in the year 1604, by one of his first works, Paralipomena ad Vitellionem, a title somewhat more modest than he was apt to assume. In this supplement to the great Polish philosopher of the middle ages, he first explained the structure of the human eye, and its adaptation to the purposes of vision. Porta and Maurolycus had made important discoveries, but left the great problem untouched. Kepler had the sagacity to perceive the use of the retina as the canvas on which images were painted. In his treatise, says Montucla, we are not to expect the precision of our own age; but it is full of ideas novel and worthy of a man of genius. He traced the causes of imperfect vision in its two principal cases, where the rays of light converge to a point before or behind the retina. Several other optical phenomena are well explained by Kepler; but he was unable to master the great enigma of the science, the law of refraction. To this he turned his attention again in 1611, when he published a treatise on Dioptrics. He here first laid the foundation of that science. The angle of refraction, which Maurolycus had supposed equal to that of incidence, he here assumed to be one third of it; which, though very erroneous as a general theorem, was sufficiently accurate for the sort of glasses he employed. |Invention of the telescope.| It was his object to explain the principle of the telescope; and in this he well succeeded. That admirable invention was then quite recent. Whatever endeavours have been made to carry up the art of assisting vision by means of a tube to much more ancient times, it seems to be fully proved that no one had made use of combined lenses for that purpose. The slight benefit which a hollow tube affords by obstructing the lateral ray, must have been early familiar, and will account for passages which have been construed to imply what the writers never dreamed of.[638] The real inventor of the telescope is not certainly known. Metius of Alkmaer long enjoyed that honour; but the best claim seems to be that of Zachary Jens, a dealer in spectacles at Middleburg. The date of the invention, or at least of its publicity, is referred, beyond dispute, to 1609. The news of so wonderful a novelty spread rapidly through Europe; and in the same year, Galileo, as has been mentioned, having heard of the discovery, constructed, by his own sagacity, the instrument which he exhibited at Venice. It is, however, unreasonable to regard himself as the inventor; and in this respect his Italian panegyrists have gone too far. The original sort of telescope, and the only one employed in Europe for above thirty years, was formed of a convex object-glass with a concave eye-glass. This, however, has the disadvantage of diminishing too much the space which can be taken in at one point of view; “so that,” says Montucla, “one can hardly believe that it could render astronomy such service as it did in the hands of a Galileo or a Scheiner.” Kepler saw the principle upon which another kind might be framed with both glasses convex. This is now called the astronomical telescope, and was first employed a little before the middle of the century. The former, called the Dutch telescope, is chiefly used for short spying-glasses.

[638] Even Dutens, whose sole aim is to depreciate those whom modern science has most revered, cannot pretend to show that the ancients made use of glasses to assist vision. Origine des Découvertes, i., 218.

Of the microscope. 46. The microscope has also been ascribed to Galileo; and so far with better cause, that we have no proof of his having known the previous invention. It appears, however, to have originated, like the telescope, in Holland, and perhaps at an earlier time. Cornelius Drebbel, who exhibited the microscope in London about 1620, has often passed for the inventor. It is suspected by Montucla that the first microscopes had concave eye-glasses; and that the present form with two convex glasses is not older than the invention of the astronomical telescope.

Antonio de Dominis. 47. Antonio de Dominis, the celebrated archbishop of Spalatro, in a book published in 1611, though written several years before, De Radiis Lucis in Vitris Perspectivis et Iride, explained more of the phenomena of the rainbow than was then understood. The varieties of colour had baffled all inquirers, though the bow itself was well known to be the reflection of solar light from drops of rain. Antonio de Dominis, to account for these, had recourse to refraction, the known means of giving colour to the solar ray; and guiding himself by the experiment of placing between the eye and the sun a glass bottle of water, from the lower side of which light issued in the same order of colours as in the rainbow, he inferred that after two refractions and one intermediate reflection within the drop, the ray came to the eye tinged with different colours, according to the angle at which it had entered. Kepler, doubtless ignorant of De Dominis’s book, had suggested nearly the same. “This, though not a complete theory of the rainbow, and though it left a great deal to occupy the attention, first of Descartes, and afterwards of Newton, was probably just, and carried the explanation as far as the principles then understood allowed it to go. The discovery itself may be considered as an anomaly in science, as it is one of a very refined and subtle nature, made by a man who has given no other indication of much scientific sagacity or acuteness. In many things, his writings show great ignorance of principles of optics well known in his time, so that Boscovich, an excellent judge in such matters, has said of him, ‘Homo opticarum rerum supra quod patiatur ea ætas imperitissimus.’”[639] Montucla is hardly less severe on De Dominis, who, in fact, was a man of more ingenious than solid understanding.

[639] Playfair, Dissertation on Physical Philosophy, p. 119.

Dioptrics of Descartes.—Law of refraction. 48. Descartes announced to the world in his Dioptrics, 1637, that he had at length solved the mystery which had concealed the law of refraction. He showed that the sine of the angle of incidence at which the ray enters, has, in the same medium, a constant ratio to that of the angle at which it is refracted, or bent in passing through. But this ratio varies according to the medium; some having a much more refractive power than others. This was a law of beautiful simplicity as well as extensive usefulness; but such was the fatality, as we would desire to call it, which attended Descartes, that this discovery had been indisputably made twenty years before by a Dutch geometer of great reputation, Willibrod Snell. The treatise of Snell had never been published; but we have the evidence both of Vossius and Huygens, that Hortensius, a Dutch professor, had publicly taught the discovery of his countryman. Descartes had long lived in Holland; privately, it is true, and by his own account reading few books; so that in this, as in other instances, we may be charitable in our suspicions; yet it is unfortunate that he should perpetually stand in need of such indulgence.

Disputed by Fermat. 49. Fermat did not inquire whether Descartes was the original discoverer of the law of refraction but disputed its truth. Descartes, indeed, had not contented himself with experimentally ascertaining it, but, in his usual manner, endeavoured to show the path of the ray by direct reasoning. The hypothesis he brought forward seemed not very probable to Fermat, nor would it be permitted at present. His rival, however, fell into the same error; and starting from an equally dubious supposition of his own, endeavoured to establish the true law of refraction. He was surprised to find that, after a calculation founded upon his own principle, the real truth of a constant ratio between the sines of the angles came out according to the theorem of Descartes. Though he did not the more admit the validity of the latter’s hypothetical reasoning, he finally retired from the controversy with an elegant compliment to his adversary.

Curves of Descartes. 50. In the Dioptrics of Descartes, several other curious theorems are contained. He demonstrated that there are peculiar curves, of which lenses may be constructed, by the refraction from whose superficies all the incident rays will converge to a focal point, instead of being spread, as in ordinary lenses, over a certain extent of surface, commonly called its spherical aberration. The effect of employing such curves of glass would be an increase of illumination, and a more perfect distinctness of image. These curves were called the ovals of Descartes; but the elliptic or hyperbolic speculum would answer nearly the same purpose. The latter kind has been frequently attempted; but, on account of the difficulties in working them, if there were no other objection, none but spherical lenses are in use. In Descartes’s theory, he explained the equality of the angles of incidence and reflection in the case of light, correctly as to the result, though with the assumption of a false principle of his own, that no motion is lost in the collision of hard bodies such as he conceived light to be. Its perfect elasticity makes his demonstration true.

Theory of the rainbow. 51. Descartes carried the theory of the rainbow beyond the point where Antonio de Dominis had left it. He gave the true explanation of the outer bow, by a second intermediate reflection of the solar ray within the drop: and he seems to have answered the question most naturally asked, though far from being of obvious solution, why all this refracted light should only strike the eye in two arches with certain angles and diameters, instead of pouring its prismatic lustre over all the rain-drops of the cloud. He found that no pencil of light continued, after undergoing the processes of refraction and reflection in the drop, to be composed of parallel rays, and consequently to possess that degree of density which fits it to excite sensation in our eyes, except the two which make those angles with the axis drawn from the sun to an opposite point at which the two bows are perceived.