PROGRESS OF METEOROLOGY IN 1889. 201 



toj;etlier with (liagrtiins, cliarls, and colored plates, 'i'lie rei>ort upon 

 the optical plienoinena Ibllowing- the eruptiou has been made by Mr. E. 

 Douglas Archibald and Hon. Rollo Russell, and occupies over 300 

 pages. The following resume of the report is taken from Xatnre. The 

 different sections coi>taiu discussions of the following topics : 



(1) The proximate cause of the abnormal tvYilights, and an explana- 

 tion, as far as was possible, of the way in which they dififered from ordi- 

 nary twilights, both in (piality and intensity. 



(2) The colored suus, large corona round the sun ami tuooii, and the 

 sk^'-haze or eruptiou cloud which evidently caused them. 



(3) The geographical distribution, the height and duration of the 

 glows, a list of analogous pheuomena on former occasions, opinions put 

 forward to account for the present series, and finally, a general analysis 

 of their cotiuection with the eruptions of Krakatoa in detail, each in a 

 separate section. 



To give some idea of the principal facts and conclusions, we will com- 

 mence with the abnormal twilights, considered as local phenomena. 



A normal sunset consists of a series of bauds of color parallel to the 

 horizon in the west in the order from below upwards — red, orange, yel- 

 low, green, blue, together with a purplish, glow in the east over the 

 earth's shadow, called the " counter-glow." As the earth's shadow 

 moves upwards towards the zenith, and passes invisibly across it, a 

 reddish or purplish glow suddenly appears above the colored layers in 

 the west, in a spot which previously appeared of a peculiarly bright 

 whitish color. This purple glow is substantially the " primary glow," 

 or more definitely ''erste purpurlicht." It is peculiar in appearing 

 above the horizontal colors, and in not extending far on either side of a 

 vertical plane through the sun and the spectator. As this glow sinks 

 on the horizon and spreads out laterally, it forms the first red sunset. 

 After its disappearance, under favorable conditions, a second edition of 

 twilight colors analagous to the first commences with a similar bright 

 spot (dammerungschein), out of which a second purple light appears 

 to be suddenly developed, and sinks on the horizon as the secondary or 

 "after-glow." 



These are the normal phases of a complete sunset, according to Dr. 

 von Bezold, and the present series appear to be abnormal only in ex- 

 hibiting certain peculiar yellow and greenish tints, a less defined bound- 

 ary of the earth's shadow, together with a much greater brilliancy, ex- 

 tension, and duration of the first, and particularly of the second, purple 

 glows. The horizontal layers were less conspicuous than usual, and 

 the abnormal extension of the purple light made it appear as though 

 there was an inversion of the usual order of tints from below upwards. 



In order to explain these and other peculiarities, Mr. Eussell starts 

 with the observed fact of a sky-haze which, in the tropics, tended to 

 transmit blue or green rays in preference to red, and assuming that al- 

 the usual elements which are included under the term "optical diffu- 



