[A25]. It has been since ascertained that Saturn has seven satellites, as is more particularly mentioned in the subsequent note. W. B.
[A26]. It was about six years after the delivery of this oration, (viz. on the 13th of March, 1781,) that Herschel discovered the Georgium Sidus. And nearly eight years and an half after this first discovery, he made two others: on the 28th of August, 1789, he was enabled to ascertain, by means of his telescope of forty feet focal length, that Saturn has a sixth satellite; and, on the 17th of September following, he found that he has a seventh. The same celebrated astronomer has since made several important discoveries. Thus, under the liberal patronage of his sovereign, has the great Herschel succeeded, by his extraordinary skill and industry in the making of very large specula, in constructing telescopes, which, in the words of the learned Mr. Vince, “have opened new views of the heavens, and penetrated into the depths of the universe; unfolding scenes which excite no less our wonder than our admiration.”
Many important discoveries (some of which are noticed in the foregoing pages of these memoirs) have been made by other eminent astronomers[eminent astronomers], since the date of Dr. Rittenhouse’s Oration; some of them, indeed, since his decease; among which are the discoveries of three new planets. W. B.
[A27]. The celebrated Huygens, who, in his Latin works, is styled Hugenius. W. B.
[A28]. Among the many eminent astronomers in the sixteenth and seventeenth centuries, mentioned by Mr. Lalande, in his Astronomie, with interesting particulars concerning most of them, the only notice he there takes of his ingenious countryman, who endeavoured to establish the theory of Vortices which he had projected, is in these words: “Descartes (René,) né en Touraine en 1596, mort à Stockholm en 1650. Sa vie a été écrite fort au long par Baillet, à Paris, 1691, in 4o.” W. B.
[A29]. The philosophy of Aristotle retained terms so very obscure, that it seems the Devil himself did not understand, or at least could not explain them; otherwise we can hardly suppose, that, when the good patriarch of Venice had summoned his attendance for this very purpose, he would have been so rude as to put him off with an answer not only unintelligible but inarticulate. See Bayle, in Art. Barbaro.
[A30]. Alluding to the experiments made in France, for determining the velocity of light; which, though unsuccessful, discovered a noble philosophical spirit.
[A31]. This prodigious velocity of light can be no argument against its materiality, as will appear from the following considerations. The greatest velocity which we can communicate to any body, is that of a cannon-ball, impelled by gun-powder; this may be at the rate of about 20 miles in a minute of time. The planet Saturn moves about 360 miles in a minute, that is 18 times swifter than a cannon-ball; and the comet of 1680, in its perihelion, moved near 56.66 times swifter than Saturn, or 990.5 times swifter than a cannon-ball. Now these are material bodies, moving with very various, and all of them exceedingly great velocities; and no reason appears why the last mentioned velocity should be the utmost limit, beyond which nature cannot proceed; or that some other body may not move 7 or 8 hundred times swifter than a comet, as light is found to do.
That the different refrangibility of the rays of light, on which their colours depend, arises from their different velocities, seems so natural a conjecture, that it has perhaps occurred to every one who has thought on this subject. To this there are three principal objections. The first is, that, according to this hypothesis, when the satellites of Jupiter are eclipsed, their colour ought to change, first to a green and then to a blue, before their light becomes extinct; which is contrary to experience. But this objection appears to me of no weight; for we do not lose sight of the satellite because there is no light coming from thence to the eye, but because there is not light enough to render it visible. Therefore at the time a satellite disappears, there is still light of all colours arriving at the eye: and though the blue light should predominate on account of its slower progress, yet the red may predominate on another account; for along the edge of Jupiter’s shadow, as it passes over the satellite, a greater proportion of red light, than of blue, will be thrown by the refraction of Jupiter’s atmosphere. The second objection is, that since the velocity of the earth in its orbit, causes an aberration of about 20 seconds in the place of a star, if the different colours of light depended on different velocities, the aberration of blue light ought proportionably to exceed that of red light, which would give such an oblong form to a fixed star as might be discovered with a good telescope. This objection is of no more force than the former. The effect ought indeed to follow, but not in a sensible quantity; for at the altitude of 70 degrees, the apparent place of a fixed star is likewise removed 20 seconds by refraction, and the very same separation of the rays must take place; yet this I think is not discoverable with the best telescope. Perhaps by uniting these two equal causes, which may be readily done, and thereby doubling the effect, it may become sensible.
The third objection arises from that curious discovery of Dollond, by which we are enabled so greatly to improve refracting telescopes. And this objection I shall for the present leave in its full force; as well against the above hypothesis, as against every other which I have seen for the same purpose.