XVII.—NOTES ON THE PHYSICAL APPEARANCE OF THE COMETS 
b AND «¢, 1881, AS OBSERVED AT BIRR CASTLE, PARSONSTOWN, 
IRELAND. By Orto Borppicxrr, Pu.D. Puare XIX. 
[Communicated by the Earl of Rosse. | 
Owing to the vacation, and afterwards to the unfavourable weather, it was not 
until July 20th that the first observation of Comet b, 1881, could be obtained, and 
only on five subsequent nights was it possible to continue observing. Thus alto- 
gether six drawings of, however, not at all equal value were made. Comet c could 
be observed on one night only, as, when clear nights again set in, it sank too low 
early in the evening, and did not rise again until morning twilight. 
All the observations were made with the reflector of 3 feet aperture, the speculum 
of which had been recently repolished. 
The following remarks on the appearance of the comets were taken from the 
observing book. The time referred to is mean time Greenwich: the powers used 
were 144 and 216. 
Comet b, 1881. 
Pl XIX, Fig. 1. July 20, 11h. 45m.—about 13h. 30m. Very clear. 
No detailed structure at all perceptible. Nucleus very bright and very well 
defined. Brightness from the nucleus very suddenly decreasing, tail rapidly and 
considerably fainter. The nebulosity in the coma (towards the apex) seems to be 
brightest. I thought sometimes I saw a darker space in the tail behind the nucleus. 
Pl. XIX., Fig. 2. July 22, 10h. 25m.—about 11h. 25m. Partially clear. 
Nucleus very bright, extended towards both sides, as shown in drawing. Tail 
rather suddenly fainter. Coma almost uniformlybright. Behind the nucleus towards 
the tail decidedly a darker lane (like a shadow, only broader than the nucleus)* ; 
hardly any trace of it visible further on in the tail. When the telescope was moved 
the coma appeared more like a fan, but when the image was kepti steady this was not 
very, if at all, perceptible. About 11h. 25m. Difference of light between “ shadow ” 
and tail very slight. No structure in coma any longer visible. Sky begins to be 
covered with a thick haze. 
In the drawing the contrast between the “ shadow ” and surrounding nebulosity 
is not quite strong enough, 
* Whenever this dark space has been observed it will be referred to as the “shadow” in the following 
notes. 
TRANS. ROY. DUB, SOC, N.S., VOL. I. 2R 
