64 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Introduction. 



This paper is prompted mainly by the many references 

 made by astronomers and astronomical publications to 

 " Baily's Beads" in cases of total solar eclipses; to the 

 "black drop" and "ligament" in transits of Mercury 

 and Venus over the Sun's disc; to the projection of a star 

 upon the bright body of the Moon at immersion or emersion 

 of occultation; to stars and the satellites of Jupiter seen 

 through the limb or body of that planet at occultation ; to 

 planets surrounded by an atmosphere of great depth; and 

 to empirical corrections to the semidiameters of the Sun 

 and Moon at meridian transits. 



We have lately examined the reports of the observers of 

 the transits of Venus in 1874 ^^^^ 1882; of the transits of 

 Mercury since 1878, and the reports of the total solar 

 eclipses since i860; and we are amazed at the crude and 

 apparently indigested explanations given for certain appear- 

 ances attending the phenomena. There w^ould seem to 

 have been a desire to seek explanations in some obscure 

 cause or causes — such as the poor character of the objective 

 used, the lack of sharpness in the limbs of the planets 

 (owing to their possessing an atmosphere), diffraction, 

 telescopic and occular irradiation, astigmatism, "sympathetic 

 attraction," etc. 



These explanations are very unsatisfactory, and the 

 experienced observer, who has observed occultations of 

 stars by the Moon or by Jupiter, meridian transits of the 

 Moon, transits of Mercury and Venus across the Sun's 

 disc, and total solar eclipses in a serene atmosphere, knows 

 that the outlines of the planets, the Moon, and the Sun are 

 remarkably sharp and steady, even under high magnifying 

 power; that the observations made under such conditions 

 are satisfactory; and that there are no extraordinary phe- 

 nomena present. 



In such an atmosphere the spurious disc of a star, large 

 or small, moves along the horizontal thread of the Meridian 

 Circle, or the Zenith Telescope, so steadily that the error 



