M.-P— Vol.. I.] DAVIDSON— APPARENT PROJECTION, ETC. 85 



occasionally reached Mercury;" leaving the inference that 

 the planet was steady. At yet another station the "planet 

 appeared with rippling fiery and black balls chasing each 

 other between the planet and the Sun." In the transit of 

 1878 one observer "was surprised at the absence of glare 

 and tremor; and the edge of the Sun's disc was clearly de- 

 fined and remarkably steady with the exception of a few 

 notches and scratches." 



We had been fortunate in our observations of the two 

 transits of Venus, and of several transits of Mercury, never 

 to have seen an exhibition of "black drop" or "ligament," 

 except at the transit of Mercury in 1891, when the atmos- 

 pheric conditions of San Francisco were very unfavorable 

 although the sky was clear; the limbs of the Sun and 

 planet were both spurious or factitious on account of exces- 

 sive vibration; and the observation was necessarily difficult 

 and doubtful. Under such adverse conditions the observa- 

 tion may be very wild, or it may turn out very good; but 

 certainly the observer cannot assign much weight to the 

 epoch noted. 



Instructions for Observing the Transit of Venus. 



In re-examining the large number of reports of the transits 

 of Venus of 1874 "^"^^ 1882, we have gathered forty or 

 fifty expressions of the observers in their endeavors to de- 

 scribe the phenomenon usually known as "black drop," 

 "ligament," or "filament." 



This need hardly be wondered at. The British Instruc- 

 tions of 1882 assert that the phenomena seen by most ob- 

 servers near the time of contact are of a complex character, 

 and extend over considerable intervals of time. These in- 

 structions surely conveyed the impression that these complex 

 phenomena were the normal physical conditions. They 

 added confusion to previous descriptions by mention of the 

 "light of the cusps;" and evidently regarded the exhibition 

 of haze, or ligament, as "the glimmering of the light of the 

 'aureole,' 'penumbra', or 'sunlight' refracted through the 



