﻿Mr. R. A. Proctor on the Zodiacal Light. 57 



the light, it would follow, precisely as in the case just con- 

 sidered, that the light would grow brighter and brighter up to 

 the very edge of the shadow. (Supposing that edge to corre- 

 spond to the extent of the earth's shadow, there would be a 

 somewhat ruddy bordering ; but up to the commencement of 

 that fringe there would be a regular increase of brilliancy.) But 

 passing over this consideration (and also the consideration that 

 the observed aspect of the zodiacal light in our latitudes is wholly 

 inconsistent with the aspect thus shown to be due to the hypo- 

 thesis we are dealing with), we may take as most favourable to 

 the hypothesis of a meteoric ring near the earth those observa- 

 tions of the zodiacal light in tropical regions which give to the 

 ordinary apparitions of the light the greatest observed extension 

 from the sun. 



We have it on the authority of Professor Piazzi Smyth that, 

 even when he observed the zodiacal light under exceptionally 

 favourable conditions (from an elevation, namely, of no less 

 than 11,000 feet above the sea-level), the western tongue had 

 completely set fully four hours before the eastern tongue began 

 to rise. Now, even if the eastern tongue were just beginning to 

 rise when the western tongue had fully set, there would still be 

 an arc of 180° between the two vertices*. But the shadow of 

 the earth would not account for such an arc as this between the 

 vertices, unless the outer part of the ring had a radius not ex- 

 ceeding ^/2 x radius of the earth (even in the most favourable 

 case of a station near to the equator) ; and with such a radius as 

 this the outer part (even) of the ring would be always invisible 

 from places having a higher northerly or southerly latitude 

 than 45°. 



And even if we set this demonstration on one side for a mo- 

 ment, it is yet obvious that, whether a ring lying relatively near 

 the earth coincided in plane with the equator or with the ecliptic, 

 or with any intermediate plane, it could not possibly exhibit 

 any approach to coincidence with the celestial ecliptic when 

 viewed from high latitudes. Further, as seen from high 

 northern latitudes, such a ring would always have a parallactic 

 displacement, causing it to lie to the south of its geocentric 

 position, and vice versa ; whereas no such association between 

 the latitude of the observer and the apparent position of the 

 zodiacal light has ever been observed, far less such a systematic 

 association as the case requires. 



It is geometrically impossible, then, that the ordinary aspect 

 of the zodiacal light can be accounted for by any theory which 

 represents it as due to a ring of light-reflecting bodies around 



* It must be remembered that each vertex, as the zodiacal light was 

 seen by Professor Smyth, lay close by the ecliptic. 



