HISTORY OF PHYSICAL ASTRONOMY.

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from the earth be at the same time assumed as the unit of distance, itwould follow from the photometric experiments of the astronomer justmentioned, that the distance of Antares, as dedueible from the quantity oflight which it emits, would be represented by 2'014, which is equal to1-6 + -414. Consequently, if the nomenclature of the stars was so mo-dified that the magnitude of Antares, as it stood in the old scale, wasincreased by ‘414, the resulting magnitude would then afford an accu-rate representation of the relative distance of the star; and the samemay be said of all the other stars whose conventional magnitudes havebeen rectified by Sir John Herschel * * * § .

Hooke appears to have been the first person who considered the modein which the visibility of the stars depends upon the telescope. In hisMieographia he remarks that the number of stars visible in a telescopewill increase with the enlargement of the aperture, for, by uniting a greatnumber of rays into one point, many stars, which from their faintnesswould otherwise be invisible, are thereby brought into view, and renderedconspicuous!. In 1717, J. Cassini alluded to the same subject in a com-munication to the Academy of Sciences , on the parallax of Sirius. Heremarked that the stars of the sixth magnitude are six times more remotethan those of the first, and hence he concluded, that with a telescopemagnifying 200 times, it would be possible to perceive stars that were1200 times more remote than those of the first magnitude];. The visi-bility of the stars is here made to depend wholly upon the magnifyingpower of the telescope. That some obscure connexion really does existbetween the two principles appears evident from the observations of sub-sequent astronomers §, but it has been no less unequivocally establishedthat the visibility of the stars mainly depends upon the magnitude of theaperture.

Lambert, in his “ Cosmological Letters,” accounts for the power of thetelescope to render small stars visible by the superior precision of theimage which it forms, when compared with that formed by the naked eye.He makes no allusion whatever to the magnitude of the aperture, but heexpressly asserts that the visibility is independent of the magnifyingpower. Michell, in the year 1767, took a more correct view of the sub-ject, making the visibility to depend on the magnitude of the aperture.Assuming the diameter of the pupil of the eye to be equal to one-third ofan inch, and roughly estimating the quantity of light lost in passingthrough the telescope, he was enabled to compare the distance of the

* The researches of Sir John Herschel do not extend to stars lower than those of thefourth magnitude. It would appear from a recent paper by Mr. Dawes {Mo. Proc. Ast.Soc., June, 1851), that the nomenclature of telescopic stars, as indicated by the obser-vations of Lalande, Bessel, and Argelander , is based upon a different principle.

+ See the work cited, p. 241.

J Mem. Acad, des Sciences, 1717, p. 260.

§ Thus Sir William Herschel, in one of his earlier communications to the Royal So-ciety , cites several instances of stars which could not be seen with a magnifying power of227, but which became distinctly visible when a power of 460 was applied {Phil. Trans.,1782, p. 92). Mr. Dawes has recently remarked that the aperture necessary to renderthe stars steadily visible to him in a telescope magnifying sixteen times, is less than thepupil of his eye, being only '15 inch in diameter, whereas he estimates that of thepupil of his eye at "25 inch (Mo. Pro. Ast. Soc., June, 1851). Now in such a caseit might be expected that the aperture of the telescope would be greater than the pupilof the eye, in order that a compensation might be effected for the light necessarily lostin telescopic observation. Is not this apparent anomaly due to the influence of the mag-nifying power, which, under certain circumstances, is favourable to the visibility of theobject, as in the instances above alluded to?