156 



NA TURE 



\_June 12, i < 



part of the rays which go to form the compound white, was 

 plainly of a different grain from the small particles commonly 

 present in the sky, for these arrest the blue rays and scatter 

 them, allowing the rays towards the red end of the spectrum a 

 freer passage, so as to impress the eye with the predominant red 

 colour of luminous objects seen through a long stretch of atmo- 

 sphere. Since the declining sun in India turned strongly green, 

 the particles competent to arrest the red rays must have ex- 

 ceeded, in the path of the rays, the ordinary blue-arresting 

 particles in quantity or power. But as the sun approached close 

 to the horizon, the lower atmosphere, by cutting off the more 

 refrangible rays, reduced the green, and sometimes caused the 

 red to predominate in the setting sun. The particles of a com- 

 mon blue haze cause the sun to set deep red. The volcanic dust 

 particles may have exceeded in magnitude the particles which 

 cause haze, and possibly the stratum may have contained par- 

 ticles which might be visible under the microscope. That this 

 dust stratum was still present in the higher atmosphere in 

 January was indicated by the greenish tinge of moon and stars. 

 It was largely composed of particles of sufficient magnitude to 

 reflect white light, for a little before sunrise the sky seemed 

 clouded over with something resembling white cirrus haze ; but 

 like a film of dust on a mirror, or the floating dust in a room, it 

 was not visible except at certain angles. Condensed vapour, or 

 ice particles in a very fine state of division, would account for 

 the persistent halo or corona of varying radius, but so also would 

 particles of transparent pumice. Assuming the red-arresting 

 stratum to have remained during the autumn and winter 

 months at altitudes from forty to twenty miles, descending 

 say iooo feet per clay during loo days, the effects observed 

 after sunset and before sunrise were only what might be ex- 

 pected to follow by reflection from the minute surfaces. In 

 the case of ordinary cirrus, the tints up to half an hour after 

 sunset are as follows : white, pale yellow, yellow, orange, 

 pink, red, deep red ; or the red only may be visible if the texture 

 be thin and the early twilight strong. With a continuous red- 

 arresting stratum, however, we must consider what influence its 

 horizontal breadth, through which the sun's rays must pass when 

 near setting, would have upon the light reflected from the west- 

 ern sky. At a height of thirty miles the sun would be shining 

 through a great length of the stratum, as viewed from the ele- 

 vated point, when it had already set on the earth immediately 

 below. At this point, thirty miles above the earth's surface, sup- 

 posing that to be the height of the stratum, the vapour of the 

 lower air would not yet be strongly exerting its influence in arrest- 

 ing the blue rays, but the sheet of dust would exert its maximum 

 power of stopping the red rays, and the light which survived 

 best, and which from the earth's surface we should see reflected 

 soon after sunset from above the western horizon, would be green. 

 The stratum being so composed as to be capable of reflecting 

 all kinds of light, but by its own action through a great breadth 

 filtering out some of the less refrangible rays, as it did more 

 powerfully in India when less attenuated, the reflected light of 

 the sun above the western horizon, and indeed towards north 

 and south as well, could not fail to be affected with an excess of 

 green. As the sun sank still lower, viewed from the height 

 of thirty miles, it would begin to be largely robbed of the 

 blue and green rays by the ordinary lower atmosphere, and 

 the next colour in the western sky would consequently 

 be yellow, which would equally be reflected by the matter 

 composing the stratum. The yellow would be the result of a 

 competition between the red-arresting upper dust and the blue- 

 arresting lower air. As the sun descended still lower, the power 

 of the ordinary vapour-charged strata would assert itself, and the 

 yellow would pass to orange, pink, and crimson, just as the 

 colour of the sun seen from any eminence commonly changes in 

 setting. The upper haze would merely reflect these naturally 

 changing colours, but the later tints would be more striking as 

 darkness increased. All the changes observed in the first after- 

 glow are thus fully accounted for by larger than ordinary sky 

 particles arresting red waves and the general mass of the stratum 

 reflecting all rays falling upon it. The secondary after-glow 

 would show similar gradations if the first were strong enough 

 to emit much light, but the red in it would be most con- 

 spicuous, for the action of the lower air in eliminating blue would 

 be more powerful than the thin veil of dust in eliminating red. 

 There was, however, a distinct greening of the eastern sky on 

 several occasions, signifying the approach of the secondary after- 

 glow. The increase of apparent brilliancy of both glows as they 

 sank westwards would of course be due to perspective. 



THE FIXED STARS 1 

 II. 



T HAVE said that the angle between the stars is measured in 

 -*■ terms of the scale, but the scale-value, in seconds of arc, may 

 change by the effects of temperature and from other causes. 



Bessel, in his researches on the parallax of 61 Cygni, deter- 

 mined by independent means the effect of temperature on his 

 scale-value, and applied corresponding corrections to his observa- 

 tions. But he also took the precaution to employ two stars of 

 comparison situated at right angles to each other with respect 

 to the principal star, so that the effect of parallax would be at a 

 maximum for one comparison star at the season of the year when 

 it was at zero for the other, and vice versa. 



But in the course of previous researches I found that there 

 were sources of error other than mere change of the temperature 

 of the air, viz. differences of temperature in different parts of the 

 instrument, and changes in the normal focus of the observer's 

 eye, which exercised a very sensible influence on the results. It 

 was necessary to devise some method by which these should also 

 be eliminated. 



Diagram _II. — Showing comparison stars employed 

 parallax of a Centauri. 



determining the 



There is a very simple means of doing this. Instead of taking 

 two comparison stars at right angles, take two comparison stars 

 situated nearly symmetrically on opposite sides of the star whose 

 parallax is to be determined — such, for example, as the stars a 

 and 3 in Diagram II. Now observe these distances in the 

 order a, 0, $, a, on each night of observation ; so that on 

 each night the observations of both distances are practically made 

 at the same instant. Then, whatever causes have combined to 

 create a systematic error in the measurement of one of these dis- 

 tances, precisely the same causes must create precisely similar 

 systematic error in the measurement of the other distance. Thus 

 if, by the regular or irregular effects of temperature or by 

 changes in the normal condition of the observer's eye, we 

 measure the distance o too great, so for the simultaneous ob- 

 servations of the distance j8 we shall from precisely the same 

 causes measure that distance too great also. 



But the difference of the distances will be entirely free from 

 all errors of the kind ; and, if the distances are not quite equal, 

 it is very easy to apply a correction on the assumption that the 

 sum of the distances is a constant. 



In Diagram II. the circle represents a radius of 2° surrounding 

 the star o Centauri. The distance of the component stars a, and a. i 

 Centauri in the diagram is enormously exaggerated for the sake of 

 clearness. Guided by the principles just explained, search was 

 made for comparison stars in pairs symmetrically situated with 

 respect to a Centauri, and otherwise favourably situated for 

 measurement of parallax. 



You will remember that from the effects of parallax all stars 

 appear to describe small ellipses about a mean position ; stars 

 near the pole of the ecliptic describing nearly circles, and those 



1 Lecture on Friday evening, May 23, at the Royal Institution, " OnRecen 1 

 Researches on the Distances of the Fixed Stars, and on some Future Problems 

 in Sidereal Astronomy," by David Gill, LL.D., F.R.S., Her Majesty's 

 Astronomer at the Cape of Good Hope. Continued from p. 137. 



