
PHENOMENA OF THE ZODIACAL LIGHT. 493 
In order to give a clearer description of what may be expected to be seen, 
than can be conveyed in words alone, I have subjoined a number of drawings,* 
both of what the zodiacal light is, and what it is not, the latter being the great 
comet of 1668 and 1843, mistaken on both occasions for the more permanent 
members of the system. The object in the construction of the drawings has been, 
in so far as it was possible to be compassed by the small skill of the author, to 
give so complete a reproduction of all the attendant phenomena, and circumstances 
of climate or country, as to enable any one who looks at them, to form a tolerable 
idea, whether any of the accompanying conditions under which the original view 
was obtained, were likely to produce an erroneous judgment in the spectator, of 
the exact form and appearance of the zodiacal light. A larger portion of the land- 
scape has therefore been introduced, than would otherwise have been altogether 
appropriate in astronomical drawings. 
A more important addition is, however, that which I was advised to make by 
my friend Mr W. A. Cape.t, viz., the insertion in the margin of the circles of 
Right Ascension and Declination, which shew what particular projection has 
been employed, and serve to identify the stars, fix the latitude in which the ob- 
servations were made, the time at the instant, and to give an idea of the dimen- 
sions of the body under discussion, and the region of the sky in which it is found. 
To represent the eastern or western portion of the sky, in their perfection, or 
as would be thrown on paper by the camera lucida, as Mr Cavett has shewn, 
the horizon should be defined by a straight line in the picture, and the E. or W. 
point must be in the middle of that line; then the eye of the spectator being 
directed toward it as such, the equator, being a great circle, will be represented 
by a straight line drawn through that point, and rising at an angle to the hori- 
zon, equal to the latitude of the place; and the meridian lines on the parallels of 
right ascension, being also great circles, must be expressed by straight lines cross- 
ing the equator at right angles; on the other hand, the parallels of declination 
being small circles, will appear as conoidal curves. <A great circle becomes a 
straight line on the picture, since it is a plane passing through the eye, and the 
common section of this plane with the planes of the picture, is a straight line. A 
small circle is a conoidal curve on the picture, because a small circle is seen as 
a cone of which the apex is at the eye, and the common section of this cone with 
the plane of the picture is a conic section. The form of the conic section will vary 
as the inclination of the cone’s axis to the plane of the picture varies. 
In all the drawings given herewith, the line of sight is seldom directed exactly 
to the E. or W. points, but generally between them and the northern or (point of 
culmination for the southern hemisphere.) Were the spectator to face the 
* On account of the expense of first-class engravings, one of the drawings only has been put 
upon metal. This one, “ the appearance of the zodiacal light at the Cape of Good Hope, in July 
1845,” will be found amongst the plates at the end of the volume. 
VOL. XX. PART III. OR 
