118 Prof. Challis on the Zodiacal Light. 



autumn months. It is evident that the light seen in the autumn 

 lies generally in the same direction from the sun as the light 

 seen in the spring ; and as the earth is at these periods in oppo- 

 site parts of its orbit, the appearances would be satisfied, so far 

 as regards northern observations, by supposing the light to be 

 of a conical or pyramidal form with the base near the sun, and 

 not to be equally and similarly extended in the opposite direc- 

 tion from the sun. To ascertain whether this is really the case, 

 I wrote, in default of available observations, to a friend resident 

 in the interior of Brazil for information respecting the southern 

 aspects of the phenomenon, and in a letter dated October 2, 

 1848, received the answer that he " had frequently remarked, 

 particularly the last few days, about an hour after sunset, a strong 

 light in the west, some 10 degrees broad, rising to the height of 

 50 degrees at least above the horizon, diminishing in breadth, 

 but not converging to a definite point." This was evidently the 

 zodiacal light stretching out from the sun just in the opposite 

 direction to that in which it is seen in northern latitudes. From 

 this observation it may be inferred that the light is symmetri- 

 cally disposed with reference to the sun, and that it is not visible 

 in northern latitudes in autumn evenings and spring mornings, 

 solely because the small inclination of the ecliptic to the horizon 

 at those epochs brings it into positions in which it must be 

 viewed through dense parts of the atmosphere, which its light is 

 too feeble to penetrate. This inference is placed beyond doubt 

 by the valuable observations of Professor C. Piazzi Smyth made 

 at the Cape of Good Hope in the years 1843, 1844, and 1845, 

 and communicated in February 1848 to the Royal Society of 

 Edinburgh (Transactions, vol. xx. p. 489). About the time of 

 the solstices the inclination of the ecliptic to the horizon will be 

 nearly the same in the morning and in the evening hours, and 

 consequently the zodiacal light, if seen at all, should be seen 

 both morning and evening at the June solstice in southern lati- 

 tudes, and at the December solstice in northern latitudes. Pro- 

 fessor Smyth has in fact recorded observations of the positions 

 of the apices on June 5 and July 6, 1845, both in the morning 

 and in the evening. The two apices are situated in opposite 

 directions from the sun, and the line joining them and conceived 

 to pass through the sun, will be at right angles to the line simi- 

 larly connecting the apices seen in spring, or in autumn, in 

 opposite hemispheres. Subsequently to my having knowledge 

 of the observations above mentioned, I found from a list of ob- 

 servations of the zodiacal light collected by M. Houzeau, and 

 published in the Astronomische Nachrichten (vol. xxi. col. 187), 

 that Cassini had taken morning and evening observations of the 

 apices on two days not far from the winter solstice, namely, on 



