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HISTORY OF PHYSICAL ASTRONOMY.

Sir William Herschel devoted considerable attention to the subject ofthe apparent diameters of the stars, but he was unable to effect any mea-surement on which he could rely with sufficient confidence. As might beexpected, he made the real apparent magnitudes of the stars to be lessthan any other astronomer had hitherto determined them to be fromobservation. On the 22nd of October, 1781, he observed the bright star «Lyra:, with a power of 0450, and, having measured the apparent diameterwith the micrometer, he found it amount to 0".3553 *. It may not be un-interesting to compare the absolute magnitude which this result wouldassign to the star, with the absolute magnitude of the sun. To this end itmay be remarked that the parallax of a Lyrre, as determined in recent timesby M. Struve, amounts to 0".261. Its distance from the earth exceeds,therefore, the radius of the terrestrial orbit in the proportion of 700,283to 1. Hence, if we suppose the sun to be transported to this distance,his apparent diameter (estimated at the mean value of 32') would bediminished in the same proportion, and would therefore be equal onlyto 0".0024. Now the apparent diameter of « Lyne, as determined byHerschel , exceeds this quantity in the proportion of 148 to 1. It follows,therefore, either that the result obtained by Herschel on this occasiondiffers widely from the true value, or that a. Lyrse vastly exceeds the sunin absolute magnitude. It can hardly be doubted that the physicalanomaly which here presents itself, is mainly due to an erroneous determi-nation of the apparent diameter of the star. Indeed, as has been alreadyremarked, Herschel does not appear to have reposed any confidence insuch fneasurements, being of opinion that the round appearance of thestars, even when seen in the most perfect telescopes, was almost whollyspurious.

When the stars are observed with telescopes of great optical perfection,furnished with high magnifying powers, they are generally found toexhibit a well-defined planetary appearance, even although the apertureshould not be reduced. There is also visible around the disk of the staran alternate succession of dark and bright rings of uniform breadth. Ithas been already mentioned that the round disk is a spurious pheno-menon, occasioned by the diffraction of light. The rings have also beensatisfactorily explained by the same principle. It is important to remark,that these phenomena would be produced in greatest perfection if the starwas merely a physical point of light. Mr. Airy, by a mathematical inves-tigation, has rigorously accounted for the appearance of the disk and sur-rounding rings, upon the principle of the mutual interference of the raysof light, and has also explained a part of the phenomenon which hadhitherto seemed very obscure, namely, the dependence of the magnitudeof the disk on the intensity of the light of the star f.

The parallax of the fixed stars having been found for a long time to beinsensible, attempts were made to determine their distances from theearth by pliotometrical principles. Assuming the stars to be equal to thesun, both in magnitude and intrinsic splendour, it is clear, since lightdiminishes in the inverse ratio of the square of the distance, that thelight of the sun will be to the light of any of the stars, as the square ofthe distance of the star from the earth to the square of the distance ofthe sun. Hence, if the quantity of light sent to us by the star bedetermined relatively to the light of the sun, its distance from the earth

* Phil. Trans., 1782, p. 147.

f Camb. Phil. Trans., vol. v., p. 283.