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HISTORY OF PHYSICAL ASTRONOMY.

asperities of the rugged chink will no longer be discernible even with atelescope, so that the edges of both chinks will assume the same smooth-ness of outline.

Galileo remarked on frequent occasions, that irradiation produced amore sensible effect according as the apparent magnitude of the objectwas less* * * § . This, indeed, is manifest to any person, when it is consideredthat the false light arising from irradiation, being the same whatever bethe magnitude of the object, cannot fail to affect small objects in a greaterdegree than large ones. It was upon this ground he explained the factthat while the aspect of the stars varied with the obscurity of the heavens,the moon continued to retain the same apparent magnitude, whether seenin the brightness of noon or in the darkness of midnight.

With respect to the cause of irradiation, Galileo imagined it to residein the eye, but he does not seem to have entertained a very decidedopinion with respect to its mode of operation. In his “ Discourse uponComets ” he conceived the irradiation of luminous objects to arise fromthe refraction of the rays of light by the humours diffused over the eye f.In his Dissertation, entitled “ II Saggiatore,” he enters more fully into thesame question. It had been supposed by some persons, that irradiationwas occasioned by objects illuminating the surrounding air, so that thecorona of light thus formed were confounded with the objects themselves.Galileo admits that when the sun or moon are near the horizon, theyilluminate the vapours through which they are seen; but he denies thatthe illumination is so intense as to render the vapours liable to be con-founded with the actual disks of either of those bodies. He then remarks,that the refraction of the moisture diffused over the eyes, causes aluminous appearance to surround the object, but being very feeble, com-pared with the original light, it does not affect the apparent magnitude.It will be seen that he here disavows the opinion previously enter-tained by him, to the effect that irradiation proceeded wholly from refrac-tion. He now considers the irradiation of luminous objects to arise from therays of light being reflected by the extremities of the eyelids, and thenbeing dispersed over the pupils |. He reproduces this opinion in hisfamous “ Dialogues on the System of the World,”§ but in a subsequentpart of the same work he ascribes the phenomena of irradiation to eitherof the two causes he had already assigned, or to some other unknowncause residing in the eye jj.

The irradiation of luminous objects being, according to Galileo , attribut-able to some ocular cause, it was clear that phenomena of this naturecould not be affected by the magnifying power of the telescope. He ex-plained by this principle the fact that the irradiation generally disappearedwhen the object was viewed through a telescope, the spurious light beingmasked by the enlargement of the apparent dimensions of the object.

The sagacity with which Galileo discusses the subject of irradiation,cannot be sufficiently admired. The least satisfactory part of his specula-tions is that relating to the physical cause of the phenomenon. It mustbe acknowledged that his views on this point are very obscure and vague.The reader cannot fail to remark that he concurs with Kepler in placingthe source of irradiation in the eye.

* Opere di Gal., tom. ii., p. 350 ; Ibid., tom. iv., p. 242.

t Ibid., tom. ii., p. 223 ; Discorso delle Comete, p. 32.

j: Ibid., tom. ii., p. 348; II Saggiatore, p. 212.

§ Ibid., tom. iv., p. 09. II Ibid., tom. iv., p. 242.