124 L. Trouvelot—Transit of Mercuvy. 
ArT. XII.— Observations of the Transit of Mercury, May 5-6, 
8; by L. TRoUvELOT. 
THE transit of Mercury over the Sun was observed at my 
Physical Observatory in Cambridge with the 64 inch refract- 
ing telescope by Merz, the full aperture being used during the 
whole time of transit. The power employed for the observa- 
tions of contacts was 158, but for physical observations higher 
powers were found necessary. Even as high as 250 and 450 
were found excellent during the afternoon. The chronometer 
was compared before and after transit with the Harvard Col- 
lege Observatory mean time clock. 
On the morning of the transit the prospects for good observa- 
tions were not promising, the s eing overcast with dense 
and continuous cirri which, however, allowed the sun to be 
seen through them most of the time. But in consequence of 
this state of the atmosphere the telescopic image appeared 
rather poorly illuminated ; although, considering the circum- 
stances the definition was fair and the image quite steady, the 
sun’s limb appearing only a little diffused. 
alf an hour before the predicted time for contact the sun’s 
surface was carefully scrutinized, but no spots were seen; and 
although a few small scattered facule were visible later, none 
could be seen at the time, owing probably to the thick va 
in the sky. No trace of the granulations of the solar surface 
could be seen. The sky forming the background to the sun 
appeared of a milky whiteness, a very unfavorable condition 
for the observation of Mercury before ingress. 
rom 225 20” till the time of contact, efforts were made to 
find the planet outside of the sun, but with no success ; although 
the telescope was directed exactly where Mercury entered the 
solar limb, and my sight must very likely have been directed 
several times where the visible planet wassituated. At 22526, 
the chronometer’s beats began to be counted, and at 224 28" 
3755, Harvard College Observatory mean time, the planet 
suddenly made its appearance, notching the sun’s limb almost 
exactly where my sight was directed at this moment. The 
suddenness of the phenomenon created some confusion in my 
mind from which ensued a delay of perhaps one or one and 4 
half seconds in my record of the time, so that most probably 
the true contact really occurred at 22 28™ 360, H. C. O. mean 
time, 
Although the contact seemed to me at first to have been 
instantaneous, yet, some unexplained phenomenon must have 
taken place immediately before I saw the black notch on the sun’s 
limb, as I distinctly remembered afterwards that my attention 
