494 PROFESSOR PIAZZI SMYTH ON THE 
northern point exactly, then the equator would be represented by a straight line, 
parallel with the horizon, and elevated above it at an angle equal to the colati- 
tude of the place. And, according to the degree to which the spectator turns 
round from the E. or W. points towards the north, the inclination of the equatorial 
line will vary, from the angle of the latitude of the place, to perfect horizontality. 
This varied inclination of the equator has been strictly attended to in all the 
accompanying cases; but it has been found advisable for simplicity in practice, 
to represent the declination circles also by straight lines, for there is hardly any 
sensible difference caused thereby in the central region of the picture, where all 
the important part of the subject to be delineated, lies; and although the con- 
figuration of stars near the borders might not be such as would exactly appear to 
the eye of a spectator, or as they should be represented by the usual rules of per- 
spective, still the amount of discordance is so extremely small, that the unassisted 
eye would hardly perceive it; and, what is more important than having a repre- 
sentation perfectly similar, in the minutest particular, to that found in the re- 
tina,—the particular projection of the sphere which was actually employed, being 
inserted in the margin, gives just as good, and rather simpler, means, than would 
have been available on the other plan, for the identification of the stars. 
The application of instrumental measurement, hereinbefore described, to deter- 
mine the phenomena of the zodiacal light, is believed to be new; and the obser- 
vations so obtained, seem to shew very decidedly, especially those of October 
1844, that numerical measures of the place of the apex of the light may easily be 
obtained,—with a probable error of not more than two degrees: so that vague 
estimations and notes of mere ideas should not be allowed to form the data in this 
particular branch of astronomy any longer. 
The general results to be deduced from the data given in the Table, are, 
1st, That the zodiacal light is a body of a lenticular form, spread out nearly 
in the plane of the earth’s orbit, and extending almost equally from the sun in 
every direction. Were the ordinary European observations made about the time 
of the spring equinox the only ones existing, we should merely be entitled to 
conclude the existence of a one-lobed projection from the sun; but when we 
combine therewith the Cape observations, we find that the body is seen all through 
the year, and on both sides of the sun, of pretty nearly the same size and shape, 
viz., a curvilinear-sided wedge, in which the light continually increases from the 
borders towards the centre of the base, or the actual position of the sun; ap- 
pearances which can only be satisfied by a lenticular body seen in section. 
2d, The zodiacal light is proved to be excentrically disposed about the sun, 
by the elongations observed east and west on the same day being different ; shew- 
ing, indeed, at various times an excentricity of from go to 76- 
3d, The zodiacal light may also be considered to rotate about the sun, and to 
be brighter in some parts than in others; because it is observed to be of different 
lengths and degrees of brightness at corresponding periods in successive years. For 
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