September, 1913. 



KNOWLEDGE. 



325 



considerably from those of others. About the 

 same time the late A. C. Ranyard, formerly editor of 

 " Knowledge," was in Sicily for the purpose of 

 observing the famous total eclipse of the Sun in 1870. 

 He examined the luminosity of the zodiacal light 

 with the Savart polariscope, and saw that this gave 

 distinct signs of polarisation, thus concluding that 

 the material giving rise to it is either composed 

 of particles so small that their diameters are com- 

 parable with the wave-length of light, or else is 

 composed of matter capable of giving " specular 

 reflection." Angstrom, of Upsala, the famous 

 spectroscopist, concluded from his observations that 

 the substance emitting this light is the same as that 

 which causes the aurora. He thought that he saw 

 the " brilliant auroral line " (5567) in its spectrum ; 

 but as the aurora is a phenomenon of far greater 

 frequency in Sweden than in regions further south, 

 his observations, through the possible confusion 

 between two coexistent phenomena, are subject to 

 considerable uncertainty. 



Acrimiz, at Cadiz, also observed a faint continuous 

 spectrum with two brilliant lines, one yellow 

 {"probably an auroral line") and another bright 

 line in the blue, more refrangible than the F line of 

 hydrogen, which he thought was identical with 

 another auroral line, but which he could not identify 

 with certainty. " As," says Professor Michie Smith, 

 " his observations were made with an instrument 

 whose dispersion was much too great to allow of the 

 visibility of the faint luminosity of the zodiacal 

 light," it seems fairly certain that this was not the 

 phenomenon which he had observed at all. 



Important observations have been made by 

 Barnard in America, Max Wolf at Heidelberg, Innes 

 at Johannesberg, Ferrari, Maxwell Hall, Backhouse, 

 and others ; but to enable us to arrive at a completely 

 satisfactory theory it is evident that we must have 

 more observations made in elevated regions, where 

 the influence of the denser part of the atmosphere 

 and other disturbing causes have less influence, as 

 well as careful measurements made over a term of 

 years. Hitherto we have had to rely mainly upon 

 observations made too near the sea level, under 

 unfavourable conditions, and of a spasmodic nature. 

 Thus it can scarcely be wondered at that there are 

 divers views and conflicting theories as to the true 

 nature of the zodiacal light. 



We have already alluded to a yet more mysterious 

 phenomenon, the " Gegenschein " (counterglow), as 

 Brorsen called it when he discovered it some sixty 

 years since. It is a faint luminosity of variable size, 

 in general more or less of the form of an oval patch, 

 but occasionally lens-shaped, lying along the Ecliptic 

 or close to it, its longitude differing from that of the 

 Sun by 180°, whence its name, from its always being 

 in exactly the opposite part of the sky. A very 

 extensive series of observations has been made by 



Mr. T. W. Backhouse, the well-known astronomer, 

 of West Hendon House, Sunderland, for more than 

 thirty years, and he has published his results in 

 Vol. II of the publications of that observatory. He 

 finds that the ordinary form of the Gegenschein is 

 nearly circular, its diameter about 7°, its centre lying 

 a little to the north of the Ecliptic (0°-75 N. lat.). 

 The Gegenschein precedes the " anti-solar point " in 

 the sky by 0°-6 of longitude at the most.* 



Professor Barnard being at Nashville, Tennessee, 

 U.S.A., during the autumn of 1883, whilst searching 

 for comets, one night noticed a feeble luminosity in 

 the sky " near Pegasus." He thought at first that 

 this was due to local illumination, although it was 

 remarkably steady. The following night it was 

 still there, and by observation of its position for 

 several nights running he found that it moved 

 towards the east along the Ecliptic at the rate of a 

 degree per day. HehadrediscoveredtheGegenschein. 

 Following up his first observations he continued to 

 make numerous examinations of the object, discover- 

 ing various changes during the course of the year. 

 Fifteen years of observation convinced him that 

 these changes are seasonal, and are repeated annually 

 in the same part of the sky. 



About the Gegenschein there is also a variety of 

 theories. Some are inclined to regard it as due to 

 " nebulous " matter rejected from the Earth, thus 

 forming a kind of tail like that of a comet. Very 

 little of such matter would be sufficient to produce 

 its feeble luminosity, which is visible only upon a 

 perfectly dark sky. If it be indeed true that this is 

 the nature of the Gegenschein, then our Earth would 

 be richer in appendages than has been hitherto 

 supposed, possessing, perhaps, a ring or rings like 

 those of Saturn (Jones's theory of the Zodiacal Light) 

 and a tail like that of a comet (the " Gegenschein "). 

 Professor Barnard, however, is inclined to consider 

 the latter as a purely atmospheric phenomenon, not- 

 withstanding his observations and the absence of 

 measurable parallax, which he admits to be a weighty 

 objection to such a view (" Popular Astronomy," 

 No. 64). Professor Searle, whose views have been 

 already referred to, considers that the zodiacal 

 light is due to the reflection of sunlight by millions 

 of minute planetary bodies and particles of " cosmical 

 dust," too small to be seen separately. When they 

 are directly opposite to the Sun in the sky each of 

 these particles shines with a "full" disc like the 

 Moon in the same position, and the vast collection 

 of small objects fully illuminated must increase the 

 general luminosity of this region. In other positions 

 less of their illuminated sides are turned towards us, 

 and therefore they are not so easily visible. Being at 

 the same time nearest to the Earth, and full when in 

 opposition, we have the Gegenschein. Mr. Evershed, 

 of Kodaikanal, in India, explains the Gegenschein as 

 produced by molecules of hydrogen and helium — the 



* Celestial longitude is the angular distance, measured along the Ecliptic (not along the Equator) from the first point of 

 Aries, in degrees and other units of angular measure. Latitude is the angular distance north or south from the Ecliptic 



towards its poles. 



