forts of Descartes himself, and of many of his followers, they have never been overcome, so that the great improvements in optical instruments have arisen in a quarter entirely different.
Descartes gave also a full explanation of the rainbow, as far as colour was not concern ed, a part of the problem which remained for Newton to resolve. The path of the ray was traced, and the angles of the incident ray, with that which emerges after two refrac tions and one reflection, was accurately determined. Descartes paid little attention to those who had gone before him, and, as already remarked, never once mentioned the Archbishop of Spalatro. Like Aristotle, he seems to have formed the design of cutting off the memory of all his predecessors, but the invention of printing had made this a far more hopeless undertaking than it was in the days of the Greek philosopher.
After the publication of the Dioptrics of Descartes, in 1637, a considerable interval took place, during which optics, and indeed science in general, made but little progress, till the Optica Promota of James Gregory, in 1663, seemed to put them again in motion. The author of this work, a profound and inventive geometer, had applied diligently to the study of optics and the improvement of optical instruments. The Optica Promota embraced several new inquiries concerning the illumination and distinctness of the images formed in the foci of lenses, and contained an account of the Reflecting Telescope still known by the name of its author. The consideration which suggested this instrument was the imperfection of the images formed by spherical lenses, in consequence of which, they are not in plane, but in curved surfaces. The desire of removing this imperfection led Gregory to substitute reflection for refraction in the construction of telescopes ; and by this means, while he was seeking to remedy a small evil, he provided the means of avoiding a much greater one, with which he was-not yet acquainted, viz. that which arises from the unequal refrangibility of light. The attention of Newton was about the same time drawn to the same object, but with a perfect knowledge of the defect which he want ed to remove. Gregory thought it necessary that the specula should be of a parabolic figure ; and the execution proved so difficult, that the instrument, during his own life, was never brought to any perfection. The specula were afterwards constructed of the ordinary spherical form, and the Gregorian telescope, till the time of Dr Herschel, was more in use than the Newtonian.
Gregory was professor of mathematics at St Andrews, and afterwards for a short time at Edinburgh. His writings strongly mark the imperfect intercourse which subsisted at that time between this country and the Continent. Though the Optics of Descartes had been published twenty-five years, Gregory had not heard of 'the discovery of the law of refraction, and had found it out only by his own efforts ;—happy in being able, by the fer tility of his genius, to supply the defects of an insulated and remote situation.
A course of lectures on optics, delivered at Cambridge in 1668, by Dr Barrow, and published in the year following, treated.of all the more difficult questions which had oc curred in that state of the science, with the acuteness and depth which are found in all the writings .of that geometer. This work contains some new views in optics, and a great deal of profound mathematical discussion.
About this time Grimaldi, a learned jesuit, the companion of Riccioli, in his astronomi cal labours, made known some optical phenomena which hid hitherto escaped observation. They respected the action of bodies on light, and when compared• with reflection and re fraction, might be called, in the language of Bacon's philosophy, crepuscular instances, indicating an action of the same kind, but much weaker and less perceptible. Having stretched a hair across a sun-beam, admitted through a hole in the window-shutter of a dark chamber, he was surprised to find the shadow much larger than the natural diver gence of the rays could have led him to expect. Other facts of the same kind made known the general law of the diffraction or inflexion of light, and showed that the rays are acted on by bodies, and turned out of their rectilineal course, even when not in con tact, but at a measurable distance from the surfaces or edges of such bodies. Grimaldi gave an account of those facts in a treatise printed at Bologna in 1665. ' Optics, as indeed all the branches of natural philosophy, have great obligations to Huy gens. The former was among the first scientific objects which occupied his mind ; and his Dioptrics, though a posthumous work, is most of it the composition of his early youth. It is written with great perspicuity and precision, and is said to have been a favourite book with Newton himself. Though beginning from the first elements, it contains a full de velopement of the matters of greatest difficulty in the construction of telescopes, particu larly in what concerns the indistinctness arising from the imperfect foci into which rays are united by spherical lenses ; and rules are deduced for constructing telescopes, which, though of different sizes, shall have the same degree of distinctness, illumination, &c. Huygens was besides a practical optician ; he polished lenses, and constructed telescopes with his own hands, and some of his object-glasses were of the enormous focal distance of 130 feet. To his Dioptrics is added a valuable treatise De Formandis Vitris.