248 TRANSACTIONS OF THE [MAT 17, 



tion of two objects with large discs ''both pointing to the sun," 

 they certainly were not the two seen by AVatson, while they were 

 in the region covered by Watson and several other observers. 

 What the precise nature of the mistake or illusion may have 

 been it is perhaps not now jiossible to discover, but I think no 

 one, unless perhaps Mr. Swift himself, now considers the ob- 

 servation important. 



While, however, the question of a "Vulcan" is now pretty 

 definitely settled, it is not at all impossible, or even improbable, 

 that there may be intra-Mercurial asteroids, and that some of 

 them may be picked up as little stars of the sixth magnitude or 

 smaller, by the photographers at the eclipse of next August, or 

 in 18^7. The sensitiveness of our i)resent photographic i>lates 

 is now many times greater than it was even in 1882. 



As to the planet Mercury, there is very little to report. It 

 "transited" the sun in May, 1878, and again in November, 

 1881, and during the transits numerous measures were made of 

 its diameter, giving results substantially in accord with the older 

 values. I have already alluded, in connection with the earth's 

 rotation, to Newcoml/s investigation of former transits of this 

 planet as establishing the sensible uniformity of the earth's rota- 

 tion. 



The planet Venus, by her transit in 1882, has attracted much 

 attention, and much interest is felt as to the final outcome of 

 the whole enormous mass of data, photographic and visual. 

 Just how long we shall have to wait for the ]niblication, seems 

 still uncertain. I have already said, however, that probably 

 these transits will never again be considered as important as 

 hitherto. 



The most important physical observations upon the })lanet 

 during the decade seem to be those of Langley, who, during the 

 transit of 1882, observed a peculiar, and so far unexplained, 

 illumination of one point on the edge of the planet's disc, and 

 those of Trouvelot and Denning, who have observed and figured 

 certain surface markings of the planet. I think I may fairly 

 mention also our Princeton observation of tlie spectrum of the 

 planet's atmosphere during the transit, and our confirmation 

 of Gruithuisen's old observation of a white cap (likely enough 

 an ice-cap), at the edge of the planet's disc, — probably mark- 

 ing the planet's pole, and showing that the planet's equator 

 has no such anomalous inclination of 50° or 60°, as stated in 

 some of the current text-books. This cap has also been observed 

 by Trouvelot and Denning. But this lovely planet is most 

 refractory and unsatisfactory as a telescopic object, apparently 

 enveloped in dense clouds which mostly hide the real surface of 

 the globe, and mock us with a meaningless glare. 



