PROGRESS OF METEOROLOGY IN 1889. 263 



objections to Professor Kiessliiig's theory of diffraction alone, wbich 

 are detailed in Section i (c), page 250, Mr. Russell's view of the origin 

 of both glows seems to be the more probable, as well as reasonable, 

 of the two. The haze stratum appears to have been capaWe of exert- 

 ing two influences : One, diffraction of the sun's rays by its smallest 

 particles, which, with the absorption and diffraction usually affected 

 by the dust and vapor present in the lower atmosphere, caused the 

 horizontal tinted layers; the other, reflection by its larger particles or 

 lamiuiie of the horizontal laj'ers, particularly of the lowest red one, when 

 the earth's shadow had arrived at about 25"^ above the western horizon 

 and into a position whence the maximum reflective effect could be seen 

 unmasked by a diflusely illuminated background. 



The question of the blue and green coloration of the sun is next dis- 

 cussed by Mr. Archibald, particularly with reference to its intrinsic 

 characteristics and physical origin. In Section vii, in which the dis- 

 tribution of the twilight glows and the blue suns on their first circuit 

 of the globe is compared, it is shown that the mean limit of the band 

 of colored suns was about 11° north and south of the latitude of Kra- 

 katoa right around the equator, while that of the glows lay 5° l)eyond 

 this on either side. Along the latitude of Krakatoa the colors were 

 mostly white or silvery, and in one or two cases coppery. The colors 

 thus evidently depended on the density of the stream, the glows ap- 

 pearing on its borders or fringes where it was less dense. A similar 

 relation to density appears from a study of the diurnal changes with 

 varying solar altitude, the sun appearing to change from blue near the 

 zenith, through green or yellow, or disappearance on the horizon. No 

 direct physical explanation of such phenomena appears forthcoming, 

 since, according to the iihysical laws enunciated by Lord Eayleigh and 

 Professor Stokes, the diffraction of light by particles of the same order 

 of magnitude as a wave length tends to sift out the shorter blue and 

 preserve the longer red waves of light. Repeated reflections by small 

 particles tend to the same result. 



It can therefore only be explained as an efl'ect of absorption, due to 

 some particular absorptive property of the materials which composed 

 the haze. The phenomenon of a blue or green sun has been observed 

 under natural conditions, many of which are quoted, and in most cases 

 where the air was filled with fine dust from -a great variety of sources. 

 It has also been artitically reproduced by Professor Kiessling with dust- 

 filled air and vapor of water, and particularly of sulphur. Several ac- 

 counts are given in section v of blue suns seen in connection with 

 former eruptions, and Mr. Whymper's observations during an eruption 

 of Cotopaxi are conclusive as to the ability of the finest volcanic ejecta 

 to cause such an appearance. The problem which still awaits solution 

 is, what was the precise nature of the particles or gases which produced 

 the absorption ? It seems probalde that they were metallic sulphides. 



Mr. Archibald next deals with the sky-haze and its peculiar effects, 



