[From the American Journal oe Science and Alps, wa>l. XWto. 1878.] 
Transit of Mercury of May 6///, 1878; by S. I. bj\ 
The following observations made during the transit of Mer- 
cury, yesterday, have some interest from the inferences to be 
drawn from them as to physical phenomena; and to devote the 
opportunity more wholly to this object, no measures of precis- 
ion were attempted, beyond noting- the times of ingress and 
egress. The principal instruments were the equatoreal refractor 
of thirteen inches (used in the early part of the day with nine 
inches effective aperture), and a polarizing solar eye-piece, which 
dispenses altogether with any dark glass, and presents objects 
in their natural colors and relative brightness. 
1 had the fortune, at ingress, of an unusually blue and trans- 
parent skv, 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 21h 52IU 39s-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 Harvard) 
in squares whose sides represented here 15‘"S, and this enabled 
—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 21h 52"» 
50s,43. First internal contact was noted when the sunlight 
could be seen unmistakably between the disc and limb at 21h 
55m 47s-25. These entries, I 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.1' 
As the disc advanced on the sun it was closely scrutinized, 
without at any time any “bright point” or “annulus” being 458 
\ P. Langley—Transit of Mercury of May 6th, 1878. 
seen. These appearances, resting as they do on much testimony, 
particularly the unimpeachable evidence of Mr. Huggins, I was 
prepared to expect, but fruitlessly looked for with powers vary- 
ing from 120 to 800 throughout the day, with the polarizing 
eye-piece, and also by projection of the image. The phenome- 
non may depend for its visibility on exceptionally good defini- 
tion, which Mr. Huggins* appears to have had; that here was 
fairly, though not unusually, good. The darkest part of the 
planet was the center, the edges being decidedly less gray. The 
cause of this gradation came out very clearly in forming a very 
enlarged image for projection, being plainly due in most part to 
minute and rapid atmospheric tremor. In moments of best defini- 
tion the surface became of a nearly uniform shade throughout.f 
The planet has been almost uniformly described as looking 
“black” in transit, but in the instrument I use (the objective 
of which was corrected by Mr. Alvan Clark), it certainly does 
not look black. The color is decidedly less red than that of 
spot nuclei, being gray, slightly inclining toward a blue, like 
that of the spectrum between F. y. G. (It may be that this 
bluish cast comes from the secondary spectrum of the objective). 
The average light from the disc in transit is very considera- 
ble, being not much less than that of some nuclei. No 
spots were present for comparison, but being engaged in photo- 
metric determinations of these and other parts of the solar sur- 
face, 1 was provided with means of comparing Mercury with 
tints which had previously been contrasted with sun-spots 
under like conditions. Absolute photometric determinations 
of the apparent light from Mercury in transit were attempted 
by projecting a greatly enlarged image (its actual diameter was 
three quarters inch as projected), on a white surface in a dark 
camera attached to, and moving with, the equatoreal. Direct 
measurements with a Jamin photometer were unsatisfactory. 
Subsequently, by another method, a trustworthy value was 
fixed for a minimum. It was thus found that the light actually 
received on the paper apparently from the so-called “ black ” 
body of the planet, at any rate exceeded eight per cent of that 
from direct sunlight, and measures taken by the thermopile and 
galvanometer showed that heat was coming from the same 
direction. 
It need hardly be said that it is impossible that Mercury 
itself should be radiating heat and light in any such degree. 
Accordingly I take these numbers as representing (with some 
possible allowance for instrumental causes) the minimum effect 
* Monthly Notices R. A. S, vol. xxix, p. 26. 
f 1 presume that even in absolutely perfect definition there would be theo- 
retically a slight gradation due to another cause, i. e.. to the greater effect of the 
edge of the planet’s disc of the inflection, referred to in a subsequent paragraph. S. P. Langley—Transit of Mercury of May Qth, 1878. 
459 
we can assign to our own atmosphere in inflecting the solar 
radiation, a subject on which data have been hitherto desirable. 
It is evident, for instance, that from the facts here stated we 
can estimate, photometrically, the intrinsic brightness of the 
corona, since it was undoubtedly this, acting as a back-ground, 
which enabled the planet, though itself involved to a calculable 
extent in atmospheric glare, to be seen before it reached the 
solar limb. 
The observations were interrupted by haze in the afternoon 
and egress was so nearly invisible that the apparent times of 
contact are not worth giving. 
Allegheny Observatory, May 7, 1878.