This instrument, though very imperfect, compared with those which have been since constructed, gave so much satisfaction, that it remained long without any material improve ment. Descartes, whose treatise on Optics was written near thirty years after the invention of the telescope, makes no mention of any but such as is composed of a convex object-glass, and a concave eye-glass. The theory of it, indeed, was given Kepler in his Dioptrics (1611), when he also pointed out the astronomical telescope, or that which is composed of two convex lenses, and inverts the objects. He did not, however, construct a telescope of that kind, which appears to have been first done by Sckeiner, who has given •an account of it in the Rosa Ursina (in 1650), quoted by Montucla.
• After the invention of the telescope, that of the microscope was easy ; and it is also to Galileo that we are indebted for this instrument, which discovers an immensity on the one side of man, scarcely less wonderful than that which the telescope discovers on the other. The extension and divisibility of matter are thus rendered to the natural philosopher almost as unlimited as the extension and the divisibility of space are to the geometer.
The theory of the telescope, now become the main object in optical science, required that the law of refraction should, if possible, be accurately ascertained. This had not yet been effected, and Kepler, whose Dioptrics was the most perfect treatise on refraction which had yet appeared, had been unable to determine the general principle which con: nects the angles of incidence and refraction. In the case of glass, he had found by ex periment, that those angles, when small, are nearly in the ratio of three to tint, and on this hypothesis he had found the focus of a double convex lens, when the curvature of both sides is equal, to be the centre of curvature of the side turned toward the object,— a proposition which is known to coincide with experiment. From the same approxima tion, he derived other conclusions, which were found useful in practice, in the cases where the angles just mentioned were very small.
The discovery of the true law of refraction was the work of Snellius, the same mathematician whose labours concerning the figure of the earth were before mentioned. - In order to ex press this law, he supposed a perpendicular to the refracting surface, at the point where the refraction is made, and also another line parallel to this perpendicular at any given distance from it. The refracted ray, as it proceeds, will meet this parallel, and the inci dent ray is supposed to be produced, till it do so likewise. Now the general truth which Snellius found to hold, whatever was the position of the incident ray, is, that the seg ments of the refracted ray and of the incident ray, intercepted by these parallels, had al ways the same ratio to one another. If either of the media were changed, that through
which the incident ray, or that through which the refracted ray passed, this ratio would be changed, but while the media remained the same, the ratio continued unalterable. It is seldom that a general truth is seen at first under the most simple aspect : this law ad mits of being more simply expressed, for, in the triangle formed by the two segments of the rays, and by the parallel which they intersect, the said segments have the same ratio with the sines of the opposite angles, that is, with the sines of the angles of incidence and refraction. The law, therefore, comes to this, that, in the refraction of light, by the same medium, the sine of the angle of incidence has to the sine of the angle of refraction • always the same ratio. This last simplification did not occur to Snelling ; it is the work of Descartes, and was first given in his Dioptrics, in 1637, where no mention is made of Snellius, and the law of refraction appears as the discovery of the author. This na turally gave rise to heavy charges against the candour and integrity of the French philo sopher. The work of Snellius, had never been published, and the author himself was dead ; but the proposition just referred to had been communicated to his friends, and had been taught by his countryman, Professor Hortensius, in his lectures. There is no • 'doubt,_ therefore, that the discovery was first made by Snellius, but whether Descartes derived it from him, or was himself the second discoverer, remains undecided. The question is one of those, where a man's conduct in a particular situation can only be rightly interpreted from his general character and behaviour. If Descartes had been uniformly fair and candid in his intercourse with others, one would have rejected with disdain a suspicion of the kind just mentioned. But the truth is, that he appears through out a jealous and suspicious man, always inclined to depress and conceal the merit of others. In speaking of the inventor of the telescope, he has told minutely all that is due to accident, but has passed carefully over all that proceeded from design, and has incurred the reproach of relating the origin of that instrument, without mentioning the name of Galileo. In the same manner, he omits to speak of the discoveries of Kepler, so nearly connected with his own ; and in treating of the rainbow, he has made no mention of Antonio de Dominis. It is impossible that all this should not produce an unfavour able impression, and hence it is, that even the warmest admirers of Descartes do not pre tend that his conduct toward Snelling can be completely justified.