.20 



NATURE 



[Sept. 26, 1889 



will be looking into the cone of darkness, and will find 

 the twilight correspondingly diminished. 



The distance from K to D in Fig. I, and from s to T m 

 diagram II., represents about 35° or 40° of the great 

 circle, or over 2000 miles on the earth's surface ; all 

 persons located in this immense region— namely, all on 

 vessels in the Caribbean Sea and Gulf of Mexico, the 

 Arabian Sea— and all meteorological observers in N orthern 

 Mexico, Yucatan, Jamaica, and Western India, should be 

 on the watch for phenomena of the following kind : — 



(i) Note when the moon's shadow becomes first or last 



The latter object is one of special interest. It is desir- 

 able that observers should carefully record the appear- 

 ance of zodiacal light for several days before and after 

 the eclipse, as also on the 22nd itself, in order that we 

 may distinguish between local atmospheric effects and 

 those due to the eclipse. During the morning dawn in 

 the region R l), and during the twilight S T, one should ob- 

 serve that the zodiacal light attains an unusual brilliancy 

 and extent ; the points to be noted are, first, how much 

 further up from the horizon, or from the sun, is the beam 

 of zodiacal light visible durmg the few minutes when the 

 cone of shadow comes between it and the observer ; 



visible by the darkness of the high cirrus, haze, or the 

 tops of cumulus clouds or the tops of mountains. 



(2) When the darkness is at its maximum, its boundary 

 appears as a curved band of colours convex to the horizon, 

 disturbing the otherwise horizontal arrangement of the 

 illumination that constitutes the dawn or twilight. Note 

 the angular distance of this boundary above the horizon 

 at several points to the right and left of the sun. Note 

 also the colours and their arrangement, both in the twi- 

 light proper and in the shaded portions. Note especially 

 how far to the right or left the disturbance of the twilight 



III. 



again, how much nearer down toward the sun does it 

 extend ; and, again, how much broader is it toward the 

 right and left 'i Does its colour appear changed ? Some- 

 times several portions of the zodiacal light appear 

 brighter than the neighbouring regions, therefore observe 

 whether, as seen through the conical shadow, these dif- 

 ferences disappear and the whole zodiacal light has a 

 uniform gradation in intensity from the sun outward 

 toward its extremity. Sometimes pulsations or waves of 

 brightness and darkness have appeared to proceed from 

 near the sun, reaching the extremity of the zodiacal light 



II. 



or dawn appears to extend. By distance, of course, is 

 meant apparent angular distance ; but if the observer is 

 so located that distant mountain-tops are visible, he 

 should note whether they also show any phenomena due 

 to the moon's shadow. 



(3) Owing to the fact that the observers at d and T 

 are looking through a portion of atmosphere that does 

 not receive its ordinary amount of light from the sun, 

 they will be able to see objects that ordinarily would 

 be hidden by the glare of twilight, such as fainter stars 

 and especially the Milky Way, and the zodiacal light. 



IV. 



in the course of five or ten seconds ; note whether any 

 such phenomena are visible. ,v •» , ^v, 



The zodiacal light has been, on g"°d authority ob- 

 served very high above the east and west horizon, so that 

 fomemain'tain'that it is continuous through t^e zenith, 

 and therefore surrounds the earth. This view ma> 

 possibly be tested by observers located anywhere near h, 

 ^quatoJ and a long way off from the path of totahty 

 Therefore everyone should keep a record of the zodiaca 

 light if he is so located as to see it at all durmg a fe^^ 



