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lHSTOKY 01*’ PHYSICAL ASTBONOMY.

much larger than she really is. In confirmation of this fact he remarkedthat even when the planet assumes the form of a very slender crescent, sheappears round like any other star, her true figure being masked by theeffulgence of her irradiation. On the other hand, when she appears pro-jected on the luminous surface of the sun, she ought to exhibit a corre-sponding diminution of magnitude, from the irradiation of the solar lightencroaching everywhere upon her disk. Upon these grounds he con-tended that the planet ought to appear considerably less on the solar diskthan she does in any other position*.

In his “ Discourse upon Comets,” he finds occasion to refer to the irra-diation of light in refuting an argument used by his opponents, which wasbased upon an absurd opinion respecting the theory of the telescope j. Itwas contended that comets are very remote bodies, because when viewedin the telescope they did not exhibit any sensible enlargement. Thisfallacy was suggested by the analogy of the fixed stars, for as these bodiesdid not appear sensibly enlarged in the telescope, and at the same time werewell known to be very remote, so it was hastily concluded that the smalleffect produced upon their appearance in the telescope was due to theirgreat distance. Galileo demonstrated by an appeal to experiment, and byreasoning of invincible force, that the element of distance did not exercisethe slightest influence on the enlargement of an object seen throughthe telescope J. The fact that some bodies were enlarged in a greaterdegree than others, was ascribed by him to the irradiation of light. Thetelescope in most cases deprives the body of the false light arising fromthis cause, so that although it does not fail to enlarge the real angulardimensions of the body in the proportion of the magnifying power, thespurious diminution may so far neutralise the actual enlargement, that theappearance presented by the body in the telescope, may not differ sensiblyfrom that which it exhibits to the naked eye. It is to be remarked, how-ever, that the irradiation exhibited by an object to the naked eye, dependsupon the darkness of the ground upon which it appears projected; while onthe other hand, since it is the object itself freed from irradiation that weperceive in the telescope, its aspect will not be affected in this ease by theground of projection. Hence the apparent magnitude of a star will vary tothe naked eye with the obscurity of the heavens, while in the telescope it willnot undergo any change. As an illustration of the truth of this remark,Galileo cites the case of Sirius, the brightest star in the heavens. A few

* Opere di Gal., tom. ii., p. 129; Istoria e Dimostrazioni intorno alle Macchie So-lari, p. 110.

+ Discorso delle Comete di Mario Guiducci, Firenze , 1619. This dissertation uponcomets, although professedly written by Guiducci, was well known at the time of its pub-lication, to have proceeded from the pen of Galileo , and it has always been included inthe successive editions of the complete works of the illustrious philosopher.

f The following passage referring to this principle, may serve to illustrate Galileo ’smode of exposing the fallacies of his opponents:—“ Place an opaque disk at a certaindistance, and directly behind it, but four or six times farther off, place a white disk of somuch greater dimensions than the other, as just to allow its circumference to be seenlike a white ring. Now, if the telescope be directed to the two disks, it ought to follow,if the more remote disk be magnified in a less proportion than the nearer one, that theformer will be altogether covered by the latter, and the ring will totally disappear. Hence,by applying the same reasoning to eclipses of the sun, it might happen that an eclipsewhich would appear partial to the naked eye, would become total by viewing it throughthe telescope; so that while with the optical instrument we found ourselves plunged in thedarkness of night, we should be enjoying the brightness of daylight with the unassistedeye !”—Opere di Gal., tom. ii., p. 221; Discorso delle Comete, p. 26.