37° 



NA TURE 



[September 16, 192: 



transplants. The report gives detailed information 

 in tabular form about the area, cost, and species of the 

 various plantations and nurseries. The positions of 

 the different State Forests and Crown Woods are 

 indicated in a sketch map. 



Four schools for apprentice woodmen have been 

 established, where sixty men received training in 

 1920-192^ These are situated at Parkend in the 

 Forest of Dean, Burley in the New Forest, Chopwell 

 in 1 lurham, and Beauly in Inverness-shire. Research 

 was carried on by six members of the Commissioners' 

 stafi ; experiments were made on the germination of 

 seeds ; on the protection of seed-beds from drought, 

 frost, and weeds ; on insect pests ; and on planting- 

 procedure. Seventy-nine sample plots, scattered over 

 England, Wales, and Scotland, were under observation 

 in September 1921. These plots will be thinned and 

 measured periodically, in order to provide data as to 

 the rate of growth and the best methods of thinning 

 plantations of the different species. 



The report concludes with an interesting account 

 of the drought of 192 1. It caused great damage in 

 England and Wales, the death-rate among newly 

 planted trees being 35 per cent. It is satisfactory to 

 note that the Commission's plantations formed in the 

 previous year did not suffer to any appreciable extent. 



The Green Flash at Sunset. 



The " Green Ray " or " Green Flash " (Rayon Vert) at 

 Rising ami Setting of the Situ. By Prof. Dr. M. E. 

 Mulder. Pp. 141. (London : T. Fisher Unwin, 

 Ltd., 1922.) 6.?. net. 



WHEN the sun sets behind a distant and clear 

 horizon, its last rays disappear with an 

 emerald green flash. The coloration is due to the 

 refraction of light in our atmosphere by which the sun's 

 image is raised through about half a degree, the eleva- 

 tion increasing from the red to the violet end of the 

 spectrum. As the violet and — to some extent — the 

 blue rays are absorbed by the layer of air through 

 whii 1 1 the light has to pass, it is the bluish-green part 

 of the spectrum that is dominant at the ultimate 

 moment of sunset. This seems clear enough and even 

 obvious. But there are always certain minds that 

 distrust the obvious — not always to the disadvantage of 

 science — and others which rebel against a common- 

 place explanation of a striking effect. Imagination is 

 always ready to supply more or less fanciful alter- 

 natives leading to controversies and correspondence in 

 51 ientific journals. 



In this manner a considerable amount of literature 

 on the " green flash " has accumulated, and this is 

 NO. 2759, VOL. I IO] 



now collected by Prof. Mulder in a volume of 140 

 pages. The book is readable and interesting. But 

 the interest is mainly psychological, depending on the 

 descriptions by which observers record their impres- 

 sions and on their knowledge of the conditions under 

 which the green flash appears. The two serious alter- 

 native explanations that have been offered to replace 

 the one based on the dispersion of light in our atmo- 

 sphere might be dismissed in one sentence. The green 

 flash cannot be the after-image in an eye fatigued by 

 the red light of the sun, because it appears at sunrise, 

 as well as at sunset ; nor can it derive its colour from 

 actual passage through the sea, because it is also seen 

 when the sun disappears behind a land horizon. 



The author aims at giving us a complete account not 

 only of everything that can be said on the subject, but 

 of everything that has been said on it. We are told how 

 observers have put their impressions into words and 

 find transcriptions of a large part of the correspondence 

 that has appeared in Nature, in the Journal of the 

 British Astronomical Association, in the Meteorologische 

 Zeitschrift, and in other publications. The same argu- 

 ments are repeated over and over again, until we feel 

 thai a horse dead and duly flogged had better be 

 buried ; this might save us from being worried by its 

 ghosts and reincarnations. 



Nevertheless, the account has its value as a chapter 

 of scientific history. We note with interest that the 

 first printed description of the green flash that can be 

 traced is contained in a novel by Jules Verne entitled, 

 " Le Rayon Vert," and published in 1882. Perhaps 

 some readers of Nature can verify the Leit-Motif 

 of the story, taken apparently from a Scotch legend, 

 according to which those who have once seen the green 

 ray acquire the power of seeing what is in the hearts of 

 others as well as in their own. 



I first noticed the green flash in February 1875 on 

 several successive mornings at sunrise while traversing 

 the Indian ocean. The appearance was so striking and 

 the explanation seemed so obvious that I took it for 

 granted I had only witnessed a common and well- 

 known phenomenon. In July 1878 during a passage 

 to the United States I directed the attention of several 

 members of an eclipse expedition to the appearance at 

 sunset ; among them was Mr. Cowper Ran yard, whose 

 subsequent views on the subject are quoted with 

 approval !>\ Prof. Mulder. I still failed to realise that 

 the effect had never received the attention of scientific 

 men, though I understand that astronomers were 

 familiar with the fact that the light of a star near the 

 horizon is drawn out into a vertical spectrum. 



If the author's investigation of the scientific literature 

 is as exhaustive as it appears to be, the first scientific 

 notice of the green flash was published only in 1885, 



