On the Physical Appearance of the Comets b and c, 1881. 241 
(next to nucleus) darkest, There seem to be more parallel dark stripes in the tail 
(Figure 7). | 
10h. Om. Slight traces of an inverted fan (towards the tail) visible as in drawing. 
Hardly any trace of the “shadow ”-line in the tail (Figure 8). 
10h. 40m. No sharp “ shadow ”-line, but general shadow (broader and less sharply 
defined). The tail seems sometimes to terminate more abruptly on the following side. 
11h. 35m. The details, as at 9h. 50m., very well defined. No “ shadow ”-line.— 
Though the above remarks contain everything necessary for the explanation of 
the Plate, still it will be useful to state, in a few words, the chief results suggested 
by them. We begin with Comet b, Pl. XIX., Figs. 1-6. 
All the observations show in common a rapid decrease of brightness from the 
nucleus towards the coma as well as towards the tail. ‘The light on the side of 
the nucleus directed towards the sun—in the coma—is brighter throughout than 
on the opposite one; the dark part on the tail-side appears sometimes rather 
well defined (as especially in Fig. 2), but never like a sharp, straight line. Its 
traces disappear rapidly a little further from the nucleus. The apex of the coma 
terminates in an uninterrupted parabolic curve, with one exception (July 27), where 
traces of a dark segment cut out of it have apparently been observed. No great 
stress, however, can be laid on this observation, as it has obviously been made 
under rather unfavourable circumstances. ‘The radius of the coma, for instance, 
appeared “rather small” (as also on August 1), a necessary effect of a hazy sky. 
Considerable changes of the comet are only shown in the shape of the nucleus 
and the light proceeding from it, where the following different phases can be 
discerned :— 
a. Nucleus round, sharp, and well defined, surrounded by a bright structureless 
halo. (July 20, and perhaps 27.) 
b. Nucleus elongated towards both sides, the light in the coma perhaps like a 
broad fan. (July 22.) 
c. Nucleus extended towards the apex like a fan, projecting into the “shadow.” 
(July 24.) 
d. Nucleus round, and a much fainter light-fan proceeding from it. (August 1.) 
This fan consisting of different rays. (July 31.) 
The two last phases differ obviously in the degree of brightness only, and the 
fioure of the nucleus in the second one (0) is probably nothing but a very wide 
opening of the light-fan. 
The situation of this fan-like emanation of light from the nucleus with reference 
to the central line of symmetry of the whole comet changes considerably, as will 
be seen by a comparison of Figs. 3, 5, and 6. Its shape is not at all symmetrical, 
as shown by Fig. 3, where the following side terminates much more abruptly, and 
by Fig. 5, where it consists of one broader part lying towards the preceding side, 
and a fainter ray towards the following one. The darker sector between them is 
