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HTSTORY OF PHYSICAL ASTRONOMY.

disbelief in the existence of any enlargement arising from this cause,or from an impression that the enlargement, which generally appeared lessin the telescope, was in the present instance totally inappreciable, in conse-quence of the immense focal length of the instrument by the aid ofwhich the measurements were effected. But whatever may have been theopinion of Newton on this point, it is certain that after his discovery ofthe unequal refrangibility of light, the subject of irradiation ceased to as-sume the same degree of importance which induced Galileo so repeatedlyto refer to it in the course of his researches, its effects being for a longtime confounded with those depending on the aberration of the telescope.

The phenomena exhibited during the transits of Venus across the sun’sdisk in the years 1701 and 1709, had the effect of again directing theattention of astronomers to the subject of irradiation. Lalande found bycalculations based on the times which Venus actually occupied in crossingthe sun’s disk during the transits of 1701 and 1769, that the apparent dia-meter of the sun, as assigned by the solar tables, was too great by 6" or 7"Du Sejour was conducted to the same result by his elaborate researches onthe annular eclipse of 1704. He found that the observations of the eclipsecould not be satisfied, except by supposing that the apparent diameter ofthe sun, as deduced from the solar tables, was too great by 9" or 6".6, ac-cording as the tables of Clairaut or those of Mayer were employed f.Lalande seems disposed to consider the enlargement of the solar diameteras due to the aberration of the telescope. Du Sejour applies the termirradiation to the enlargement, asserting, however, at the same time, thathe does not mean thereby to announce any opinion respecting its physicalorigin.

In 1782, while Sir William Herschel was engaged in a series of experi-ments for the purpose of determining the apparent diameter of the planethe had recently discovered, he noticed an interesting fact illustrative ofthe irradiation of light. The method he employed for measuring theplanet’s disk consisted in comparing its image as seen through a telescopewith an artificial lucid disk, formed by making a circular aperture in paste-board, and covering it with transparent paper illuminated behind by aflame. The image of the planet was observed in the telescope with theright eye, while the lucid disk was viewed directly with the left, its dis-tance being made to vary until it appeared exactly of the same size asthe image in the telescope. Hence, the linear diameter and distanceof the artificial disk being known, it v'as easy to deduce its apparentdiameter, and by combining this datum with the magnifying power of theinstrument, the apparent diameter of the planet was also readily ascertained.In order to ensure an accurate comparison of the disks, Herschel pre-pared a series of circles, varying in diameter from 2 to 5 inches, andincreasing successively by tenths of an inch. He remarked, that as thebrightness of the image in the telescope varied with the altitude of theplanet and the magnifying power of the instrument, so it was necessary, inthese experiments, to alter in a corresponding degree the brightness ofthe lucid disk, for he found that the magnitude of its apparent diameterwas very sensibly affected by this cause. In fact, having placed severalof the artificial disks together and illuminated them at the same time,he found that a very small quantity of additional light wus necessary, in

* Mem. Acad, des Sciences, 1770, p. 403.

+ Ibid., 1775, p. 365.