
PHENOMENA OF THE ZODIACAL LIGHT. 497 
of the two objects, however, will probably convince every one, that Cuarptn’s 
Persian expression “ niazouk,” or in French “ petit lance,” which was applied by 
the Persians to the phenomenon they saw, could only be considered as at all 
suitable in the case of the comet’s tail. 
In 1683, the subject was taken up by Donic Cassini, and to him belongs the 
merit of first scientifically investigating the laws of the phenomenon, determining 
its cosmical nature, and giving it the appropriate name of the zodiacal light. His 
series of observations, extending over nearly six years, is still unrivalled; and if 
he is not correct in all his conclusions, it is chiefly because his observations were 
almost entirely confined to his own high northern latitude; and were therefore 
affected to a great and unknown extent by circumstances of climate and geogra- 
phical position. He had much wished to eliminate these effects by means of ob- 
servations made in the southern hemisphere, but unfortunately was not able to 
obtain any; and indeed those which have been made by the author, and recorded 
in this paper, are perhaps the first which have been published and brought to 
‘bear on the theory of the subject. 
Cassint’s conclusions were, that the zodiacal light is a flat luminous ring en- 
circling the sun, nearly in the plane of his equator, and is therefore seen always 
more or less in profile, and perfectly so at two periods of the year, April and August, 
when like Saturn’s ring, and for similar reasons, he supposed it to vanish to our 
sight; while the nonvisibility at any period between these two months, he con- 
sidered to be produced mainly by the overpowering effect of the lengthened sum- 
mer twilight. But these ideas, on being tested by the Cape observations, com- 
pletely fall to the ground; for during the whole period of invisibility to Cassrnt 
(caused in reality by the lengthened twilight of summer in his northern hemi- 
sphere), the phenomenon was most visible at the Cape, as winter then prevails in 
the southern hemisphere; and, indeed, the very reverse effect from that expected 
by Cassini should follow, when a transparent and oblate luminous ring is viewed 
in profile, for it will then be seen at its brightest, on account of all the infinitely 
small light-giving particles being brought closer together; so small are they, that 
they can by no means be distinguished separately, or when thinly scattered over 
the sky, but only make themselves sensible to the eye, and the telescope, when 
they are crowded together in a smaller space. The idea, moreover, of the zodiacal 
light being in the form of a ving at all, is discountenanced by the observed appear- 
ances, they being all conformable to the phenomena which would be afforded by 
a thin lenticular body, excentrically situated and revolving about the sun. 
Cassinr’s friend, M. Fario, made observations of the zodiacal light about the 
same time, as did also M. Kircu, and Ermmarr, and Mr DErRHAM. 
But the subject was not carried further, until taken up by Marran, in 1731, 
He was rather wild in his notion of the manner in which the body was formed, 
VOL. XX. PART III. 6s 
