ZODIACAL LIGHT. 



sphere, without the possibility of being seen or observed. It 

 may be objected that they would be illuminated by the sun's 

 light, as the moon and planets are, and must thus be rendered 

 visible. Their extreme minuteness, however, affords a satis- 

 factory explanation why they are not visible by the light which 

 they reflect. Compared with the planets, as Sir J. Herschel 

 observes, which are visible in our most powerful telescopes, 

 rocks and stony masses of great magnitude and weight would 

 be but as the impalpable dust which a sunbeam renders 

 visible as a sheet of light, when streaming through a narrow 

 chink into a dark chamber. It has nevertheless been affirmed 

 that occasions have been recorded on which the sun's light, at 

 noon-day and in an unclouded sky, has become sensibly 

 obscured during certain intervals of time, which has been 

 explained by the supposition that at such times a flight of 

 meteoric stones were passing between the sun and the earth, so 

 as partially to intercept the sun's light. 



A stony mass which would weigh an hundred tons, however 

 strongly illuminated by the sun's light, would not be visible at 

 the distance of eight hundred or a thousand miles. 



23. Sir John Herschel supposes it probable that the sun is 

 surrounded by a mass of nebulous matter of greater or less 

 extent, such as is seen around many of the stars, and that the 

 phenomenon called the zodiacal light, as well as meteoric stones 

 and shooting-stars, are mere manifestations of this nebulous 

 matter. 



The zodiacal light is explained by the supposition of an oval 

 spheroid of nebulous matter surrounding the sun, the larger 

 diameter of which coincides with the solar equator. 



In fig. 2, s s represents the sun's equator, and A B A' B' the 



oval mass of surrounding nebulous matter shown by its section 

 made by a plane through the axis of the sun, the section by a 

 plane through the sun's equator being a circle whose diameter 



is A A'. 



157 



