54 



KNOWLEDGE. 



February, 1913- 



ing the pipette. 

 It may be men- 

 tioned here that 

 certain other 

 liquids may be 

 used instead of 

 orthotoluidine ; 

 for e x a m pie , 

 anisol produces 

 spheres when run 

 into water at 

 15°C. But the 

 beautiful colour 

 of orthotoluidine, 

 combined with 

 its insolubility- in 

 water, and the 

 absence of other 

 objectionable 

 features, renders 

 this liquid more 



* 



Figure 62. Forming a sphere of 

 orthotoluidine. 



Figure 63. The detached sphere 

 floating under water. 



suitable than any 

 other, either for 

 producing con- 

 trolled drops or 

 spheres. It is, 

 moreover, re- 

 latively a cheap 

 liquid, and should 

 be welcomed by 

 all who are in- 

 terested in this 

 fascinating 

 branch of physics, 

 as it enables 

 phenomena, for- 

 merly difficult 

 and troublesome 

 to observe, to be 

 demonstrated in 

 the simplest 

 manner. 



COLOURS AND THEIR CHANGES AT SUNSET ON A 



TROPICAL ISLAND. 



By MAXWELL HALL, M.A., F.R.A.S. 



Jamaican Government Meteorologist. 



In connection with the researches into the constitution of the 

 upper regions of the atmosphere, the following notes of the 

 colours and their changes usually seen during the first part of 

 the year at the Kempshot Observatory, Jamaica, may prove 

 to be useful. The colours and their changes are best seen 

 from January to April, before the summer heat causes great 

 ascensional movements of vapour, and before there are many 

 particles of water in the air, which give rise to the gorgeous 

 colours seen in the autumnal months ; the steadiness of the 

 air throughout the nights of the early part of the year is very 

 remarkable, so much so that even stellar photometry is 

 rendered easy and pleasant work. The latitude of the obser- 

 vatory is 18° 25' north; the elevation is 1773 feet; the mean 

 temperature from January to April is 71°; the dew-point is 

 65° ; so that the elastic force of aqueous vapour is 0-62 inch, 

 and the humidity 81. The position of the observatory on a 

 range of hills commands a view round the horizon ; and the 

 hills and country round about it are clothed with thick vege- 

 tation, so that there is but little dust in the air. Under these 

 favourable circumstances notes were made which have been 

 condensed into the following. 



Ten minutes before sunset a pink band appears on the 

 eastern horizon : it can usually be traced all round the 

 horizon ; but it soon disappears except in the eastern half, 

 where it grows stronger in colour and rises up from the 

 horizon as though on pivots at the north and south points. 

 Underneath it a dark band of a leaden blue colour appears. 



At sunset this dark band is about li° broad; and the pink 

 band resting on it is about 4J° broad. 



Five minutes after sunset a faint pink glow, which has 

 spread rapidly upwards from the pink band, reaches the 

 zenith. And looking above the western horizon where the 

 sun went down, there is a large patch of white light ; its 

 boundaries can hardly be defined, but they give the idea of 

 an equilateral triangle, whose sides are about 45°, its base 

 being parallel to and somewhat above the horizon. Of course, 

 this white light and the pink glow are easily seen by contrast 

 with the blue and bluish-violet of the remaining parts of the 

 sky. 



Ten minutes after sunset the dark blue band, still resting 

 on the eastern horizon, is 4° or 5° broad, and is now at its 



darkest; it tapers off gradually to the north and south points 

 of the horizon. The pink band resting upon it is now 

 15° broad, and similarly tapers off. The pink glow has 

 reached the white patch in the west, and begins to replace it. 



Fifteen minutes after sunset the dark band is 63° broad, 

 and begins to fade away in grey colour. The pink band has 

 gone. The pink glow has entirely crossed over to the west, 

 where it is now at its strongest somewhat below the top of the 

 former white patch. 



Twenty minutes after sunset the pink glow in the west, 

 having approached the horizon, disappears. 



Thirty-five to forty minutes after sunset the colours of the 

 spectrum are best seen in the sky to the west, all parallel to 

 the horizon ; red on the horizon, and indigo or violet 

 25° above. This completes the colour-changes, the subject 

 matter of this article ; but the following notes are interesting 

 and may be added. 



Fifty to sixty minutes after sunset the zodiacal light appears 

 about 20° or 25° above the western horizon. 



Seventy-three to seventy-six- minutes after sunset, the band 

 of twilight along the western horizon, or the twilight arc, as it 

 is called, coincides more or less with the base of the zodiacal 

 light, which makes it necessary to discriminate between them 

 in ascertaining the greatest breadth of the zodiacal light near 

 the horizon and the greatest duration of twilight. 



The angular measures referred to above were taken by a 

 sextant from which the telescope had been removed. The 

 general correctness of the whole was proved by frequently 

 watching the colours and their changes at sunrise, when the 

 whole sequence of phenomena was repeated, but, of course, in 

 a reverse order. 



On the plains near Kingston, Jamaica, there is much dust 

 in the air near the ground, and the phenomena are not for the 

 most part so well seen ; but in the early mornings I have 

 there seen the dark blue band in the west as distinct or 

 even more distinct than at the observatory. 



It would appear that the surface which reflects the faint 

 pink glow is about fifteen miles above the surface of the 

 earth ; but it would be advisable to know how far these 

 phenomena have been observed in other uniform climates 

 before attempting their explanation. 



