286 British Association for the Advancement of Science. 
those great stumbling blocks of comets, is really superb. How- 
ever, what I have now to relate, refers to a very singular and in- 
structive fact in its physical history. I saw the comet for the 
first time after its perihelion passage, on the night of January 25. 
Mr. Maclear saw it on the 24th. From this time we of course 
observed it regularly. Its appearance at first, was that of a round, 
well-defined disc, having near its center, a very small bright ob- 
ject exactly like a small comet, and surrounded by a faint nebula. 
This nebula, in two or three more nights, was absorbed into the 
disc, and disappeared entirely. Meanwhile the disc itself dilated 
with: extraordinary rapidity, and by measuring its diameter at 
every favorable opportunity, and laying down the measures by 
a projected curve, I found the curve to be very nearly a straight 
line, indicating a uniform rate of incréase ; and by tracing back 
this line to its intersection with its axis, I was led, at the time, to 
this very singular conclusion,—viz. that on the 21st of January, 
at 2 h. p.m. the disc must have been a point,—or ought to have 
no magnitude at all! In other words, at that precise epoch some 
very remarkable change in the physical condition of the comet, 
inost:iays ‘cdmnieticed Well !-all this was speculation. But 
here comes the matter of fact I refer to, and which, observe, 
was communicated to me no longer ago than last month by 
the venerable Olbers, whom I visited in my passage through 
Bremen, and who was so good as to show me a letter he had just 
received from M. Boguslawski, Professor of Astronomy at Bres- 
lau, in which he states, that he had actually procured an observa- 
tion of that comet on the night of the 2Ist of January. Well 
then, how did it appear?—why, as a star of the sixth magni- 
tude,—a bright concentrated point, which showed no disc, with 
a magnifying power of 140! And that it actually was the comet, 
and no star, he satisfied himself, by turning his telescope on that 
point where he’had seen it. It was gone! Moreover, he had 
taken care to secure, by actual observation, the place of the star 
he observed ; that place agreed to exact precision with his com- 
putation ; in short, that star was the comet. Now, I think this 
observation every way remarkable. First, it is rernarkable, for 
the fact, that M. Boguslawski was able to observe it at all on the 
21st. This could not have been done, had he not been able to 
direct his telescope poifit-blank on the spot, by calculation, since 
it would have been impossible in any other way to have known 
