Zodiacal Light. 149 



of a series of observations on the zodiacal light, among which 

 are the following : — 



1. The comparatively few nights in the year when, in our 

 climate, the sky is cloudless, and the atmosphere sufficiently 

 clear to afford good observations on a light so feeble and 

 diffuse. 



2. The low angle which the zodiacal light makes with the 

 horizon for the greater part of the year while it is visible. 



3. The presence of the moon, which entirely effaces it, and 

 occasionally, for long periods, the presence of Venus or 

 Jupiter, and sometimes of both planets. The light of Venus, 

 especially, is often so bright, and the planet is so situated in 

 the midst of the zodiacal light, as greatly to interfere with 

 observations. Hence a number of years are necessary of 

 diligent attention to the phenomena of this light, in order to 

 become well acquainted with its habitudes and laws. Nor 

 can I pretend to have made the best possible use of the oppor- 

 tunities afforded for viewing it, during the six years that my 

 attention was directed to it. On the contrary, my observa- 

 tions were often interrupted by ill health, and other causes 

 beyond my control. Still, they were sufficient to convince 

 me that ray previous knowledge of this body was exceedingly 

 defective, and my notions of it very erroneous ; and the same 

 may justly be said of most or all of the descriptions and 

 graphic representations of it given in works of science. 



I will therefore, first, attempt an accurate description and 

 representation of the zodiacal light. 



Since the direction of this body is oblique to the circles of 

 diurnal revolution, and since it appears only immediately 

 before or immediately after the sun, and therefore more or 

 less of it falls within the twilight, consequently, its appear- 

 ances are very different in different latitudes, being seen best 

 of all in the tropical regions, where its direction always makes 

 a high angle with the horizon, and where the twilight is short ; 

 and being scarcely visible in such high latitudes as London 

 and Edinburgh, except near the time of the equinoxes. Hence 

 British writers who have attempted a description of it, have 

 usually given one that is altogether vague and inaccurate. 

 The lower latitudes of our place of observation (4F 18' 30") 



