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



atmosphere of Venus across the dark space between the 

 cusps." And further, if a "pure geometrical contact" (which 

 is the only line that can be given for contact) is frustrated by 

 haze, shadow, ligament, or black drop, then the last marked 

 discontinuity of the illumination of the Sun near the point of 

 contact is the epoch to be recorded if it "is distinctly recog- 

 nized as independent of mere atmospheric tremor." 



The American Instructions were mainly based upon the 

 appearances of 1769; and declared the " atmosphere of 

 Venus clearly illuminated." They refer to the difficulty of 

 observing the moment of tangency on account of imperfec- 

 tion of vision, irradiation produced by the Earth's atmos- 

 phere, and imperfections in the telescope; and then 

 mention the difficulties to be expected by atmospheric 

 undulation. 



One of our eminent astronomers accepts the explanation 

 of Lalande, that the phenomenon of "black drop," etc., is 

 caused by irradiation at the bright object; and that this irra- 

 diation arises from a number of causes, imperfections of the 

 eye, imperfections of the telescope, and the softening effect 

 of the atmosphere upon a celestial object when seen near 

 the horizon. He likens it to a narrow and less bright false 

 edge around the bright object; and says that this band will 

 appear narrower the better the telescope and the steadier 

 the atmosphere. He furthermore says that in the observa- 

 tions of the transit of Venus of 1874, with the improved 

 instruments, very few of the experienced observers no- 

 ticed any distortion at all. Later on he adds that "in the 

 varied forms presented 'when the air is not still,' we recog- 

 nize all the peculiar appearances described by the observers 

 of 1769." In 1874 ^^ Mexican observers at Yokohama 

 reported to us a decided exhibition of "black drop." 



Colonel Tennant has remarked of the black drop phenom- 

 enon, " there is no doubt in my mind that the outer part of 

 the Sun is never free from the result of outstanding astigma- 

 tism." We have astigmatism of the human eye, and astig- 

 matism of the telescopic objective and ocular, but we do 

 not understand to which he applies the term, if to either of 



