262 PROGRESS OF METEOROLOGY IN 1889. 



sion " were preseut, viz., diffiaction, refraction, aud reflection, describes 

 what should be the effects, (1) assuming a haze composed of opaque par- 

 ticles, and (2) one composed of very thin reflecting plates into which 

 condition a large proportion of the pumice ejected from Krakatoa is 

 shown^to have been transformed. His conclusion is that the distinctive 

 features of the Krakatoa glows were due mainly to reflection from these 

 fine laminre, of rays already tinted in a certain order by diffraction 

 through the dust of the haze layer and the lower atmosphere, as well 

 as by the selective absorption which ordinarily takes place in the more 

 humid horizontal layers near the earth's surface. The direct as well as 

 diffuse reflection by the plates and by the opaque dust, (which lay, as 

 Mr. Archibald has shown in Section iv, at a height of from 50,000 to 

 100,000 feet,) of rays tinted in succession, as both the direct and reflected 

 twilight boundaries followed the descending sun, and the peculiar trans- 

 missive quality of the stratum for the more refrangible rays, appear to 

 afford a reasonable explanation of the peculiar silvery glare, the unusual 

 coloring, and the unusual extension of the purple glows. 



It is admitted that diffraction played an important part, as it does 

 in ordinary sunsets (Lommel, for example, attributes all the red tints 

 to this cause) ; but both in this section and those that follow, many 

 considerations are urged against the view held by Professor Kiessling 

 that the development of the primary glow is chiefly due to diffraction, 

 while the secondary glow is as confidently asserted to be due to reflec- 

 tion. One of the principal objections to the reflection hypothesis in 

 explanation of both the ordinary, as well as the present extraordinary, 

 development of the purple glow is its limitation at first to a narrow 

 band, a fact which cannot be explained by absorption, and which is 

 equally at variance with Fresnel's law of reflection from small globular 

 dust, which would be equal in all directions. On the lamina, and par- 

 ticularly the vitreous lamina assumption however, it is intelligible, 

 since the maximum reflection would then be like that from the sea, in 

 the vertical plane through the sun and the eye. 



Moreover, the richly-colored and prolonged secondary glows, which 

 were the most characteristic feature of the Krakatoa twilights, are 

 shown by Mr. Archibald, when dealing with their secular duration, to 

 have reached a distinct minimum when the large diffraction corona 

 round the sun, from Professor Ricco's observations, appeared at its 

 greatest brilliancy, while the curve of their duration, representing Dr. 

 Riggenbach and Mr. Clark's observations, shows that they never again 

 reached the same brilliancy or duration as in the two or three months 

 immediately succeeding their first appearance in Europe. Both these 

 facts aid the conclusion arrived at by Mr. Russell and indorsed by 

 Professor Kiessling, that they were reflections by the haze stratum 

 of the primary glows. But if these were reflections, the question 

 naturally arises, Why not the primary also? And until more effective 

 arguments are brought against this view, as well as Professor Ricco's 



