418

HISTORY OF PHYSICAL ASTRONOMY.

the glass employed in viewing a luminous object, exercises a material in-fluence on the enlargement arising from this cause. The observation ofDe Barros was called in question by several of his contemporaries, butwhen the extraordinary phenomena of a similar nature witnessed in morerecent times are taken into consideration, there seems no reason to doubtthe authenticity of his statement.

Plantade remarked that, during the transit of 1736, the disk of Mer-cury, when projected upon the sun, appeared to he surrounded by a lumi-uous ring. A similar phenomenon has been witnessed during severalsubsequent transits. MM. Schroeter and Harding, who observed it onthe occasion of the transit of 1799', describe it as a nebulous ring of adark tinge approaching to violet colour*. A ring agreeing with thisdescription was noticed by Dr. Moll, of Utrecht , during the transit of1832 j-. On the other hand, many persons who observed the transits abovementioned, did not perceive any indications of a ring around the planet, norhave the observations of more recent transits of the planet served to con-firm the existence of such a phenomenon. It is therefore very probablya spurious appearance depending upon some optical cause.

A curious phenomenon noticed by Schroeter and Harding on the occasionof the transit of Mercury which occurred May 7, 1799, consisted in theappearance of two small spots of a greyish colour on the disk of theplanet j. A similar phenomenon was witnessed by Dr. Moll during thetransit of 1832, except that in this instance only one spot was visible§.During the transit of 1848, a spot was also seen on the planet’s disk bythe Rev. J. B. Reade and Mr. Dell. It is described as a greyish spot,shading off indefinitely on all sides from the centrej|. It appears, also,from a statement made by Captain Sir Edward Belcher , R.N., to Prof.Powell, that it was seen in Sir James South’s great refracting telescope,the aperture of which (11.95 inches) had been reduced by a diaphragmoutside to 3 inches IT.

Prof. Powell has proposed an ingenious explanation of the above-men-tioned phenomenon, founded upon the principle of the diffraction of light **.It is a consequence naturally flowing from the theory of the diffraction ofthe object glass of a telescope, as explained by Mr. Airy, that a smallopaque disk, projected upon a luminous surface, should not only undergoa diminution of apparent magnitude, but should also exhibit one or morebright concentric rings in the interior, and if the disk be very small, it isnot difficult to conceive that the rings might condense into a bright spotin the centre. Prof. Powell verified this result experimentally, by viewinga small opaque disk, on a ground glass, strongly illuminated behind, witha telescope having its aperture reduced to one-fourth of an inch. Withinthe disk he distinctly perceived a ring concentric with it, which contractedinto a spot in the centre when the disk was very small.

Nothing can be more satisfactory than the agreement between theresult of the above-mentioned experiment and the phenomenon observedduring the transit of 1848. So far the explanation of Prof. Powell ap-pears to be unexceptionable. Unfortunately, the same degree of con-sistency does not present itself when the question relates to the phenome-non of a similar nature witnessed on previous occasions of the transit ofMercury. Thus, although Dr. Moll saw a small spot on the disk of the

* Mem. Ast. Soc., vol. vi. p., 116. t Ibid. + Ibid.

§ Ibid. || Monthly Proc. Ast. Soc., vol. ix., p. 23.

f Mem. Ast. Soc., vol. xviii., p. 88. ** Ibid., p. 87.