498 PROFESSOR PIAZZI SMYTH ON THE 
viz., by particles thrown off from the sun, in consequence of the rapidity of his 
rotation; nor was he very happy in his name of the “Sun’s atmosphere,” by 
which he led both himself and others to reason upon it, as if it were proved to be, 
and actually was of a kindred nature with the earth’s atmosphere. His conclu- 
sions, however, that the whole of the luminous body was of a lenticular form, 
nearly in the plane of the earth’s orbit, somewhat excentric with regard to the 
sun, and, indeed, with a rotation about that luminary, seem to be remarkably 
good. And his opinion, so far as the lenticular shape is concerned, is also held 
by Ouzers and by Sir J. Herscue, both of them observers. 
OLsERs in a letter to HumMBotpT in 1833, says, “‘ What you tell me of the 
changes of brightness in the zodiacal light, and the causes to which, in the tropics, 
you ascribe such variations, has excited my interest the more, because I have been 
for a long time past particularly attentive every spring to this phenomenon in our 
northern latitudes. I, too, have always believed the zodiacal light to rotate; but 
I assumed it (contrary to Potsson’s opinion, which you communicate to me), to 
extend the whole way to the sun, increasing rapidly in intensity. The luminous 
circle which in total eclipses shews itself round the darkened sun, I have sup- 
posed to be this brightest portion of the zodiacal light. I have satisfied myself 
that the light is very different in different years, sometimes for several years being 
very bright and extended, and in other years scarcely perceptible. I have not 
myself been able to observe the sudden fluctuations in the light, probably on ac- 
count of the faintness with which it appears to us in this part of the world. You 
are certainly right in ascribing the rapid variations in the light of celestial objects, 
which you have perceived in the climate of the tropics, to changes taking place in 
our atmosphere, and especially in its higher regions. This shews itself in a more 
striking manner in the tails of great comets. Often, and particularly in the 
clearest weather, pulsations in the tails of comets are seen to commence from the 
head or nucleus as the lowest part, and to run in one or two seconds through the 
whole extent of the tail, which, in consequence, appears to lengthen several de- 
grees, and contract again. That these undulations, which engaged the attention 
of Rosert Hooks, and in later times of ScuroprER, and Cutapnt, do not take place 
in the cometary tails themselves, but are produced in our atmosphere, appears evi- 
dent if we reflect that the several particles of these cometary tails (which are many 
millions of miles in length) are at very different distances from us, and that the 
light from them can only reach our eyes at intervals of times which differ several 
minutes from each other. I will not attempt to decide, whether what you saw 
on the banks of the Orinoco, not at intervals of seconds, but of minutes, were 
actual coruscations of the zodiacal light, or whether they belonged solely to the 
upper strata of our atmosphere. Nor can I explain the remarkable lightness of 
entire nights, or the anomalous increase and prolongation of this light in the year 

