S. P. Langley—Transit of Mercury of May 6th, 1878. 457 
4 
Art. LXV1L—Transit of Mercury of May 6th, 1878; by S. P. 
LANGLEY. 
in their natural colors and relative brightness. 
I had the fortune, at ingress, of an unusually blue and trans- 
parent sky, and aided by this, saw with the polarizing eye-piece 
the entire disc of Mercury outside the sun about one-half a 
minute before first external contact. Presumably it might 
have been seen even earlier, had not time been lost in searching 
for it, through lack of means to designate the precise position- 
angle, the position filar-micrometer not being adaptable to this 
eye-piece. After a pause to verify the reality of the phenome- 
non by revolving the eye-lens, etc., the chronograph key was 
struck at 21 52m 398-45 Allegheny mean time, to record the 
observation. As this was really made earlier, and the disc was 
seen throughout its circumference, it seems clear that the 
coronal back-ground is bright enough to produce this effect at 
least fifteen seconds of arc from the solar limb, and in spite of 
the atmospheric glare. . 
As a partial substitute for the filar micrometer, there was in 
the field a glass reticule, ruled (by Prof. Rogers, of hapa 
in squares whose sides represented here 15°’8, and this enable 
—not a measurement—but a fair comparison to be made of the 
apparent size of the planet before and after it entered on the 
sun. The contrast was striking, as on a back-ground very little 
brighter than itself its diameter was, if anything, greater than 
one of the sides of these squares, while as soon as It entered on 
the sun it seemed to shrink by more than one-fifth of this. 
First external contact was noted on the chronograph at 21» 52™ 
505-48. First internal contact was noted when the sunlight 
could be seen unmistakably between the disc and limb at 21° 
55m 478-95, These entries, 1 believe to have been made in both 
cases nearly two seconds late. The limb just at second contact, 
was steady. I saw no “black drop” or “ ligament” 
As the dise advanced on tbe sun it was closely scrutinized, 
Without at any time any “bright point” or “annulus” being 
