59- 



NATURE 



[January 30, 19 13 



sincerity cannot fail to be inspiring to anyone 

 who will be at pains to understand it. It would 

 I)e a mistake to criticise the earlier half of the 

 book as if it were a treatise on the cubic surface; 

 it is the author's assembling- of his materials for 

 the constructions which follow, and the very want 

 of elaboration which it occasionally exhibits is 

 a proof, if an incidental one, of the independence 

 with which the author has carried out his research. 

 Perhaps the analytical investigation of the double- 

 six theorem, which occupies pp. 16 to ig, is 

 an extreme case ; a geometrical proof might have 

 been given, though the author's is simple and self- 

 contained. 



The book is accompanied by reproductions of 

 elaborate diagrams, carefully drawn to scale. 

 They would have been more interesting if not so 

 much reduced in size. There is a long biblio- 

 graphy of the general literature in regard to the 

 cubic surface, which is likely to be useful. Under 

 1902 there should certainly be the entry, " Bezieh- 

 ungen der allgemeincn Flache dritter Ordnung 

 zu einer covarienten Flache dritter Classe," by 

 Th. Reye, Maih. Ammlen, Bd. Iv. Also the 

 paper of G. Kohn, " Ueber einige Eigenschaften 

 der allgemeinen Flache dritter Ordnung," \]'iener 

 SHoungsherichfe, Bd. cxvii., p. 66, should be 

 referred to, and a recent paper by Prof. W. 

 Burnside in the Camb. Phil. Proceedings, on 

 double-sixes with projective transformations. 

 Mediainisch-clicniiscJies Laboratoriunis-Hilfsbuch. 



By Dr. Ludwig Pincussohn. Pp. xi + 443. 



(Leipzig: F. C. W. Vogel, 1912.) Price 



13.50 marks. 

 OxE cannot say that Dr. Pincussohn 's book fulfils 

 any real need, seeing that laboratory guides of the 

 same nature are already numerous. The author 

 was formerly assistant to Prof. Albderhalden, 

 and is well known as an original worker in the 

 field of physiological chemistr}'. The book he has 

 produced is a very good one, and is specially 

 useful because of the tables of physical and 

 chemical data which occupy its last hundred pages. 

 The introductory chapters deal with general 

 chemical methods, and the remainder, as the title 

 indicates, with that portion of the science which 

 it is the fashion to call bio-chemistry. The analy- 

 tical and other methods of research selected are 

 up-to-date, and are described in a clear and in- 

 teresting wa)'. W. D. H. 



Books that Count. A Dictionary of Standard 

 Books. Edited by W. Forbes Grav. Pp. xx 

 + 3i5 + hiii. (London: A. and C. Black, 1912.) 

 Price s-^- net. 

 Of the fourteen sections into which this dictionary 

 is divided, one deals with science, and some others 

 are concerned with such kindred subjects as edu- 

 cation, geography and travel, philosophy and 

 sociology. The sections are necessarily incom- 

 plete, for the editor intends his lists chiefly for the 

 use of young students and ordinary readers. The 

 work should, however, be very valuable for refer- 

 ence to notable books in many departments of 

 knowledge. 



NO. 2257, VOL. Qol 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.] 



Luminous Halos surrounding Shadows of Heads. 



'I'llE plienomenon refLrred to in a note in Nature 

 of December 12 (p. 419), as observed in rice-tields of 

 Japan, can also be seen on grass when' the sun is 

 low in the sky. The presence of dew, I believe, 

 increases the intensity of the halo, bu: it is perfectly 

 distinct also on dry grass. 



If the grass surface is near to the observer, a faint 

 halo is seen to surround the shadow of his head, 

 and this is more easily perceived if he is moving than 

 if standing still; my attention was indeed first 

 attracted to the phenomenon when bicycling. 



In this mountain region I have frequently seen the 

 halo projected on a grassy slope a mile or more 

 distant, and under these conditions it appears as a 

 circular or elliptical patch of light without the central 

 shadow, the diminution of intensity due to the 

 penumbral shadow of one's head being, of course, 

 quite inappreciable at such distances. It is difificult 

 to determine the size or shape of this patch, owing 

 to irregularities in the brightness of the background; 

 but I have been able to compare it with the nearly 

 full moon rising immediately above it, and should 

 judge it to be at least 2° in diameter, and probably 

 elliptical in shape with the long axis vertical. The 

 light appears to emanate from the grass itself, which 

 apparently reflects more light in the direction of 

 incidence than in other directions ; it is certainly not 

 due to dust or haze in the intermediate column of air. 

 I am unable to say whether a smooth rock surface 

 would give the same appearance, but a dense white 

 cloud certainlv does so, with the addition of a faintly 

 coloured rine: surrounding the white patch. This I 

 presume is allied to the " Brocken spectre," seen when 

 the illuminated cloud or fog is near to the observer. 



The analogy of this elliptical bright patch opposite 

 the sun with the Gegenschein is so striking' that one 

 cannot help believing both to be due to the same 

 cause, and that matter outside the earth's orbit and 

 beyond the limits of the earth's shadow reflects more 

 sunlight in the direction of incidence than in other 

 directions. That the Gegenschein usually covers a 

 much larger angular area than the 2° patch seen on 

 these hills may be accounted for by the much more 

 favourable conditions in which it is seen, with a dark 

 and uniform sky-background. J. Evershed. 



Kodaikanal Observatory, South India, 

 January 4. 



Exactly a month ago to-day, in the Betul district, 

 Central Provinces, I had set out on field work at 

 dawn, with my colleague, Mr. H. Walker, and two 

 chaprasis (Indian servants). I happened to be watch- 

 ing our shadows as we passed along the edge of a 

 field of young green wheat, when, to my surprise, 

 I noticed a halo of light round the shadow of my own 

 head and neck. Looking at the other shadows, 1 

 was still more surprised to see that only my shadow 

 was invested with this halo. "l directed the attention 

 of Mr. Walker and the chaprasis to the phenomenon, 

 and found that each could see a halo round his own 

 head only. Whilst we were investigating the matter 

 our camp passed on the march, and inquiries made 

 both from our servants and from local people showed 



