iS. Newcomb on observing the coming Transits of Venus. 79 
der uncertain the results of observation, we shall inquire how 
its injurious effects may be obviated. 
The first question can be decided only by comparison of the 
observations of different observers upon one and the same phe- 
For such comparison I shall select the observations 
of the egress of Mercury on the occasion of its last transit over 
the disk of the sun. This selection is made for the reason that 
this egress was observed by a great number of experienced ob- 
servers with the best instruments, while former transits, whether 
of Venus or Mercury, have been observed less extensively or ata 
time when pasbtiodl: astronomy was far from its present state of 
ss and that the transit in question woul Ser: fur- 
nish much better data of judging what we might ex 
fare cbservations The comparison was made in the follow: 
selected from the “ Astronomische Nachrichten,” 
the s "ibaay Notices _ the Royal Astronomical Society,” and 
U 
tact. Next is given the aperture of the seiensope, and the mag- 
nifying power employed in the observation. Lastly, we present 
a es of the phenomena of contact as described by the 
obse 
The lettin V. C., (vanishing contact,) indicate that the thread 
of light broke by becoming paws thin, so that there was 
no irradiation = D. (black do the plan 
ters b. jadioats the appearance of the 
ligament, as shown in fig. . , 
R. R. Indicates that the ime is that of the rupture of the 
thread, but that it cannot be inferred from the statements of oa 
observer whether the thread had or had not a sensible thickn 
per es aoe before breaking. 
parent contact, as 3. 
When there is nd Diack foe the letters V. C. soars fon 
the same meaning. The letters are therefore employed daly 
when there is reason to beliens from the remarks of the ob- 
server that the cusps were rounded. 
nee rons a that ees! observer described the phenomenon 
as instan 
The letters b and g refer to the defin the first in 
that it was bad, the second that it w: wri — 
When there is no indication in thise column the observer gives 
no information respecting the phenomenon. 
