A simple explanation suggested itself at the same time to Cassini, and to Olaus Roëmer, a Danish astronomer. The former, however, withheld his theory, being uncertain whether it would be confirmed by its holding good also in regard to the other satellites, whose elements were as yet too little known to warrant any conclusion. The latter, less scrupulous, broached at once the idea that the difference is owing to the velocity, inconceivable indeed, but yet finite, with which light travels: and that it is precisely such as to occupy about fourteen minutes in traversing the diameter of the earth's orbit. He communicated an account of the whole investigation to the Academy of Sciences, in 1676.

Maraldi felt a certain degree of difficulty in admitting this explanation, observing, that a similar effect ought to follow, in a less degree, according to the position of Jupiter with regard to his aphelion; but observation was not then accurate enough to detect such an effect.

More modern observations have shown that it is actually the case; and have also extended the result to all the satellites. The caution, however, both of Cassini and of Maraldi is perfectly justified on the soundest principles of inductive science; and while we cannot fail to regard with more immediate satisfaction the bold announcement of Roëmer, yet we ought, perhaps, to esteem still more highly the sound philosophy of Cassini, which at once enabled him to grasp so happy an explanation, founded on a great principle of nature, now for the first time evinced by observation, and yet to resist the temptation of announcing it as a discovery, because it still wanted a full and legitimate induction to its establishment.

Cassini was certainly one of the greatest astronomers of his age; he was born at Nice, in 1625, and died in 1712. He determined the rotations of several of the planets on their axes by means of their spots. He gave accurate measurements of the ring of Saturn, and of the flattened form of Jupiter's disk. He greatly improved the

tables of refraction, and, from a more correct notion of its amount, was enabled to make corresponding corrections in the estimate of parallax. He discovered that singular phenomenon, the zodiacal light, or luminous train, sometimes seen extending upwards from the sun, when on the horizon, and which, to this day, has remained without explanation. He completed the theory of the moon's libration, by showing that her axis of rotation is not perpendicular to the ecliptic, but slightly inclined, and that the nodes of the lunar equator always coincide with those of the orbit. This explained satisfactorily what had been before observed, that the period of the inequalities of the libration coincided with the revolution of the nodes of the orbit.

Optics, as indeed all branches of the natural sciences, are under great obligations to Huyghens. This science, however, seems to have been that which most occupied his mind. His Dioptrics is a work, the greater part of which was composed in his youth, but which was not published till after his death. It is written with great perspicuity and exactness, and is said to have been a favourite book with Newton. Though commencing with the first elements, it contains a full developement of all the details of the construction of telescopes: a department in which the author was also practically eminent. He polished lenses and constructed telescopes with his own hands, and has appended to his Dioptrics the results of his experience in a tract, "De formandis Vitris." He gave enormous lengths to his telescopes. Some of his object glasses are now in the possession of the Royal Society, of 130 and 150 feet focal length. Such instruments were very unmanageable; to render them less so, Huyghens adopted the plan of dispensing altogether with a tube, mounting the object glass on the top of a lofty pole or building, and turning it in the

requisite direction by machinery, whilst an eye-piece was applied below. There was, however, a twofold advantage obtained by these great lengths.

The magnifying power of a telescope depends upon the relative focal lengths of its object and eye glass: the greater the ratio of that of the former to that of the latter, the greater the magnifying power. But this does not (in theory at least) involve the consequence, that the absolute focal length of the object glass should be very great. There is, however, practically, an advantage in this respect; but the main object was dependent on another principle. The fact had already been well understood, that, in oblique refraction, light is separated into colours; and any small portion of a convex lens is,' in fact, a prism, so that the rays proceed to the focus, separated into the prismatic colours, which occasions the image formed at the focus to be edged with a fringe of colours, and also rendered indistinct, owing to the want of convergence of those coloured rays at the same point: but it was soon found that the degree in which this takes place is independent of the focal length of the lens, and so long as its diameter (or aperture, as it is termed) remains the same, the degree of colour will be the same. Hence, by increasing the focal length to a great extent, the edge of colour remains the same, while the image (with a given eye glass) is greatly magnified in proportion or the coloured edge may be made to bear an insensibly small ratio to it, and thus the inconvenience be almost got rid of. We shall afterwards see why these constructions are now no longer resorted to.

The subject of colours in the refraction of light had attracted the attention of Dr. Barrow, then Lucasian professor of mathematics at Cambridge; but the theory he gave was very unsatisfactory and unphilosophical. It is, however, highly probable that its promulgation may have been the immediate occasion of directing the attention of Newton to the subject. Barrow treated of the mathematical parts of optics with all his powerful

ability, and discussed some of the most difficult problems relating to the subject, which then engaged the attention of geometers, in his lectures delivered in 1668, and published in the following year.

The invention of the reflecting telescope exhibits a modification of the same theoretical principle as that on which the refracting telescope is constructed; in an abstract point of view, perhaps, even simpler and more obvious than the latter; at any rate such as must have immediately suggested itself, as far as the principle is concerned but to reduce it to practice required some further contrivance.

Rays of light from a distant object falling upon a concave reflector, would give an image in its focus which might (in theory) be magnified by an eye-glass; but how could the eye be placed there to see it without intercepting the light? The use of the object glass, or, in this case, the reflector, is simply to collect a great quantity of light: hence the cutting away a small hole in the middle of it will not materially interfere with its office. A second small reflector, placed facing the large one, will also intercept only a small quantity of the light. The eye being placed with an eye-glass at the back of the great reflector opposite the hole, may then receive the image thrown back by the small reflector, which has received from the large one the rays going to its focus to form that image: thus the difficulty is surmounted.

This invention was made by James Gregory, professor of mathematics at St. Andrew's, and afterwards for a short time at Edinburgh. He gave a description of it in his Optica Promota in 1663. It has been since known by the name of the Gregorian construction. Cassegrain afterwards modified it by making the small reflector convex instead of concave.

The Optica Promota contains also many important investigations relating to other parts of optics, especially the formation of images by lenses. The author like.

wise directed his attention to the advantages which would theoretically result from having a reflector, whose .section should not be the arc of a circle, but of a parabola; in which case, by a property of that curve, all rays falling parallel to the axis would be accurately brought to convergence at the focus. He devised methods for forming such surfaces; but the practical difficulties encountered were so great that no advantage could be derived from the idea. Subsequent artists have experienced the same difficulties; and all attempts to prosecute such a plan have long since been abandoned; and after all, perhaps, the advantages have been overrated, since a very slight deviation from exact parallelism to the axis, would occasion considerable aberration from the true focus.

Gregory was a man of highly acute and original mind; and in a remote situation supplied the want of intercourse with the scientific world from the powerful resources of his own genius. Having no communication with the Continent, he had never seen the works of Snell or Des Cartes; but he deduced for himself the law of refraction, by an independent investigation.

Double Refraction.

Erasmus Bartholinus, in 1669, first observed the singular fact, that a small object seen through a transparent crystal of the Iceland spar appeared double. He pursued the subject, however, but little further than to give some very general description of it. It was evident that the ray was divided into two within the crystal, following different laws of refraction.

Huyghens soon after directed his attention to the phenomena of the doubly refracting crystal; by a series of accurate measurements, he determined the directions assumed by the two rays under all varieties of direction of the incident ray. One of the rays was found to follow the ordinary law of the sines in the plane of incidence; the other followed a variable law of deviation,