Febeuaky 11, 1910] 



SCIENCE 



229 



posing the tail of a comet presumably are 

 excessively minute, any meteoric trails they 

 may produce on coming in contact with the 

 atmosphere must be small. However, it would 

 be well, at the proper time, to watch for them 

 with a telescope pointed nearly vertically and 

 focused for a distance of from 100 to 150 

 mUes. Presumably only faint scintillations, 

 probably entirely too faint to be seen, need be 

 expected, but only by such observations can 

 we know definitely just what does or does not 

 take place. 



10. Bishop's Ring. — After the explosion of 

 Krakatoa, and also after that of Mount Pele, a 

 faint reddish brown ring of the coronal type 

 was seen about the sun. Its inner radius was 

 about 12°, and its outer approximately 22°. 

 It was due, almost certainly, in both cases, to 

 finely divided matter thrown up to great alti- 

 tudes and from there spread widely over the 

 earth. The mean radius of these particles, 

 assuming them spherical in shape, has been 

 calculated to be about equal to the largest 

 visible wave-length. They were therefore ex- 

 cessively minute, and it is possible that after 

 passing through the tail of a comet something 

 of this kind may be seen; at any rate, careful 

 observation should be made for it, after such 

 an event, by those of exceptionally sensitive 

 eyes. Such observations are best made with 

 the sun hidden behind an opaque object. 



11. Color of the Sun. — The color of the sun, 

 as is well known, depends upon the size and 

 number of solid or liquid particles through 

 which it is seen, and therefore may, pos- 

 sibly, be temporarily modified on our passing 

 through a comet's tail. 



12. Atmospheric Polarization. — This phe- 

 nomenon depends mainly upon the scattering 

 of sunlight by any minute particles in the 

 atmosphere. The percentage of the polarized 

 to the total sky light at any part of the sky, 

 say where the polarization is a maximum, or 

 90° from the sun on the vertical circle passing 

 through it, is a function of the dust content 

 of the air. This percentage therefore should 

 be carefully noted during our supposed coming 

 passage through the tail of Halley's comet, as 

 should also the positions of the so-called neu- 



tral points of Arago, Babinet and Brewster — 

 the first especially, as it is the easiest observed 

 and most accurately determined. 



It might also be advisable to observe the 

 polarization percentage of different colors, by 

 the aid of suitable screens, since this depends 

 upon the size of the particles that scatter the 

 light. 



13. Twilight Phenomena. — Twilight colors, 

 and the gamut of changes through which they 

 run, clearly are dependent upon the dust con- 

 tent of the atmosphere, as was strikingly evi- 

 dent after the eruption of Krakatoa, and 

 therefore might, possibly, afford some informa- 

 tion in regard to the tail of any comet through 

 which the earth may pass. 



14. Luminous Clouds. — After the eruption 

 of Krakatoa there was seen for many years, 

 but only in latitudes of 45° or more, faintly 

 luminous clouds of, seemingly, great altitudes. 



It is not at all certain that these so-called 

 clouds were due in the least to the volcanic 

 eruption; but still they should be closely 

 looked for at the time of and after our pas- 

 sage through a comet's tail, since they might 

 be modified by the material thus picked up. 



15. Numler of Dust Particles in the Air. — 

 The number of dust particles, especially in the 

 outer portions of the atmosphere, may be 

 greatly increased by the passage of the earth 

 through the tail of a comet. Therefore it 

 would be well to count the particles of dust 

 per cubic centimeter say of air on the tops of 

 high mountains, and in samples obtained by 

 sounding balloons, before and just after the 

 time of our entrance into the tail of Halley's 

 comet. 



16. Zodiacal Light. — While our knowledge 

 of the zodiacal light, of the nature and loca- 

 tion of the material to which it is due, and 

 how this material is rendered luminous, is 

 practically nil, it seems quite possible that its 

 real or apparent brilliancy may be greater 

 during our passage through even so rare a 

 substance as the tail of a comet. Therefore 

 the details of this phenomenon too should be 

 recorded, at the proper time, by those so situ- 

 ated as to observe it to good advantage. 



17. Gegenschein. — But little is known of the 



