66 REVIEWS—DONATI’S COMET. 
‘Oct. 8. The nucleus to-night is decidedly brighter than on the 
6th, and is preparing to throw off a new envelope.” 
“Oct 9. A new envelope, H., has been thrown off, as predicted 
last evening.” 
The normal process seems to be a brightening of the nucleus, then 
an envelope in contact with it; the nucleus becomes fainter, and the 
envelope spreads, becomes “ mottled” or ‘‘curdled” with intermit- 
tent jets and lumps of luminosity, its form a sharply curved outline 
towards the sun and running round more than a semi-circle into two 
cusps behind the nucleus. Gradually it rises and the dark space in- 
tervenes between it and the head, broken however sometimes by the 
jets; its light fades as it gets higher, till it begins to crumble away at 
its vertex, and gradually disappears down to its cusps which are the 
last to melt into the general haze. The interval between the disen- 
gagement of the successive envelopes varied from 4 to 8 days, and the 
velocity of each diminished as they expanded, so that they closed on 
each other in the higher regions, and the puzzling circumstance of a 
devided dark spot occurring in one of them, was of use not only in 
identifying that envelope, but in showing that there was no rotation 
round the axis in it. Curiously enough, the dark spot was repeated 
in the same relative position in the following envelope.* It is not 
* We quote the following from Prof. Bond’s snmmary of results .— 
“ At first they (the envelopes) presented a variety of aspects, but as they expanded they. 
tended to conform with a normal type, the light becoming more evenly disposed and the 
outline more symmetrical. For a few days the surface was closed on the side opposite the 
sun, although here and there penetrated by streams rising into the tail, principally from the 
cusps on either side. As it expanded, the discharge became general, but was always most 
considerable from the outside, thus forming the asymptotic brauches below the nucleus. 
The curve on the side towards the sun ina completely formed envelope was very nearly 
circular for 60° to 80° on either side of the axis. This was originally the brightest and best 
defined region, but it was also the first to fade away, the material being evidently trans- 
ferred to the outlines below the parallel of the nucleus, which remained in the sight long 
alter the upper portions had disappeared, and finally driven off into the tail. The process of 
dissipation furnishes a satisfactory explanation of the branches of the tail, which are sim- 
ply the continuation of the older envelopes merged together and undistinguishable from each 
other excepting near the nucleus. In this view the dark hollow of the axis represents the 
region not fully supplied from the envelopes, while they retained their closed or partially 
closed surfaces. . . . . After reference to the dark arcs interposed between adjoining 
envelopes and the bright marginal rims of the latter, the subject of the dark and bright 
spots on their surface is taken up. Several results of considerable importance have been: 
derived from the discussion. Among them are,—1st. A degree of permanence in the inter- 
nal distribution of the substance of the envelopes retained for a long interval after their 
ejection from the nucleus. 2nd. That their diversified axpect, especially the isolation of 
bright masses, cannot be explained as a mere optical effect produced by the intersection or 
separaticn of streams of Juminous matter passing out continually from the nucleus into the 
tail. 3rd. The nearly permanent direction maintained by the spots relatively to the axis of the 
tail proves that there was no sensible rotation of the envelopes, excepting in a sense always 
preserving an unaltered aspect towards the sun. 4th. That there was no Sensible oscillatory 
