On the Zodiacal Light. 61 



clouds, when that luminary is near the horizon, and are cal- 

 led by seamen, shrouds and backstays, from their resem- 

 blance to those appmtenances of ships. These beams, 

 which by the laws of perspective, appear to diverge from the 

 sun, are, in fact, parallel, and when seen of considerable 

 length, they of course appear arcs of great circles. When 

 the sun is in, or a little below the horizon, they are often seen 

 to converge to a point diametrically opposite to the sun, and 

 have no relation to the sun's equator. The beams are seldom 

 seen far from the vanishing point. I have seen them almost 

 every day, in certain places, within the tropics. They are 

 however often very faint, and can be seen only by a practiced 

 eye, and as in looking at a celestial nebula or the tail of a 

 comet, are best seen with the eyes partly closed, or by look- 

 ing a little to one side of the object. These beams are pro- 

 duced in the same manner as those in a room where there is 

 smoke or dust, and the sun shining in through the window. 

 The vanisliing point of the solar beams is seen in the sea, 

 when transparent and smooth, and the sun shining at least 

 60° or 70° above the horizon. Then by looking over a ves- 

 sel's side, opposite to the sun, the spectator will see aroimd the 

 shadow of his own head, only a kind of corona.* The same 

 phenomenon may likewise be seen in a stratum of fog when 

 the sun and observer are in certain positions, as when seen 



* This has been called the apotheosis of travellers. (V. Malta Brun.) The 

 appearance is caused either by rain, fog, or spray, on the same principles as the 

 common rainbow. A line drawn from the centre of the sun passing thi'ougb 

 the eye of the spectator who has his back to the sun, passes through the centre 

 of the bow. Consequently, if the spectator be elevated above the horizon, as 

 when on a high mountain, he may see the entire bow encircling the head of his 

 shadow. 



Let A be a spectator stand- 

 ing on the brow of a high 

 mountain ; let B represent the 

 sun ; then C will be the bow, 

 both parts of which will be 

 visible to the spectator; and 

 being projected on the subja- 

 cent plain, or upon the clouds, 

 along with his shadow, it will 

 appear to encircle his head 

 like a corona. 



What Mr. Leslie calls zodiacal light are phenomena included under the 

 multiform appearances othalos. La Place has an article on " Zodiacal light," - 

 but limits it to a peculiar appearance soraetilnes from about the vernal equi- 

 nox.— ia Place's System of the World. 



