June 50, 1910] 



NATURE 



517 



higher alcohols, the carbohydrates, the fats and waxes, 

 the phosphatides, the vegetable proteins, and the 

 animal proteins. The volume concludes in the middle 

 of an article by the editor on the disintegration pro- 

 ducts of the proteins, characterised by much work of 

 a patient and laborious t}-pe. The reader is deserted 

 in the middle of a sentence, with "'moglichst" for 

 his last word, but it may be confessed that one can 

 wait for the next issue of the fascinating narrative 

 \i-ith more patience than one was able to com.mand 

 in earlier days of reading serial Hterature. 



The articles in this part are of unequal value and 

 exhaustiveness ; that on the phosphatides is much too 

 short; very valuable are the three articles, on sugars 

 by B. Tollens, that on glycogen by K. Grube, and 

 especially that on the proteins of the vegetable world 

 by T. B. Osborne, which is a model of what an article 

 on practical methods for the laboratory ought to be. 



Such articles as these make the work essential in a 

 biochemical laboratory, but it is a pity that they 

 cannot be purchased as monographs in the particular 

 subjects. Benjamin Moore. 



THE LIGHT FROM THE SKY. 

 Meteorologische Optik. By Prof. J. M. Pernter. 

 Section IV. Pp. i-xvii, 559-799. By Felix M. 

 Exner. (Vienna and Leipzig : W. Braumiiller, 

 1910.) 



'T'HIS volume is the fourth part of the late Prof. 

 J- Pernter's work on meteorological optics, a notice 

 of the earlier parts of which appeared in Nature on 

 April 18, 1907. It was undertaken in March, 1908, by 

 Dr. Exner at Pernter's request, at a time when the 

 latter 's condition of health not only made it impos- 

 sible that he should complete the work himself, but 

 •prevented him from giving any material assistance or 

 advice in its preparation. The scope of the section 

 was clearly indicated in the original plan of the work, 

 and as regards the method of presentation Exner has 

 successfully followed that of the preceding sections; 

 but he had no notes to assist him, and the volume 

 must therefore be regarded as Exner's work, except 

 in so far as he has utilised, in some portions, Pern- 

 ter's previously published papers. 



In pursuance of the general scheme outlined in the 

 previous notice above referred to, this fourth sec- 

 tion is devoted to the discussion of those phenomena 

 which are due to the action of the minute particles 

 of all kinds which are always present in the atmo- 

 sphere, among which must be included the gaseous 

 molecules themselves. Thus the first two chapters 

 deal with the colour and polarisation of sky-light, the 

 third treats of the loss of light in passing through 

 the atmosphere and the general brightness of day- 

 light, while in the fourth chapter is given a brief 

 account of what are called the phenomena of twilight, 

 the optical effects associated with sunrise and sunset. 



Of all the many interesting problems the discussion 

 of which falls under the head of meteorological optics, 

 that of the colour of the sky, with the associated 

 questions as to the polarisation of sky-light, its in- 

 tensity and composition, and the effects of atmospheric 

 absorption, is perhaps the most fascinating. Less 

 NO. 2122, VOL. 83] 



striking only because not exceptional, in the sense in 

 which this adjective applies to the appearances dealt 

 with in the earlier sections, even for the unscientific 

 obser\er the everyday recurrence of the phenomena 

 fails to diminish their appeal to his artistic sense and 

 imagination. For the physicist, the satisfactory 

 explanation of all the main features, apart from 

 the quantitative uncertainty in the details which is an 

 almost inevitable consequence of the complexity of the 

 conditions, must be ranked among the triumphs of 

 science, and constitutes one of the most beautiful 

 applications of the wave theory of light. 



The \-iew has long correctly been held that the 

 colour of the sky is due to the presence in the atmo- 

 sphere of suspended particles, and the explanation was 

 rendered more certain by the experiments of Brucke 

 in 1853 and of Tyndall in 1868 on the colour and 

 polarisation of the diffused light from artificially 

 "clouded" media. The first exact account as to the 

 manner in which the particles produced the effects 

 observed was, however, given by Lord Rayleigh in 

 1871, in his paper on the light from the sky, its 

 polarisation and colour. It was there shown that the 

 presence in the atmosphere of particles of dimensions 

 small compared with the wave-length of light would 

 give rise to secondary radiations of intensity inversely 

 proportional to the fourth power of the wave-length, 

 and completely polarised in the plane at right angles 

 to the direction of the primarj- radiation from the sun. 

 In this secondar>% diffused radiation, the short waves 

 would therefore greatly preponderate, and the colour 

 seen would be blue or violet, while the long waves 

 would be the more readily transmitted, and the 

 primar}' radiation seen through such an atmosphere 

 would tend to be orange or red. At the same time, 

 Rayleigh disposed of Clausius's theor>^ that the 

 phenomena were due to the presence in the atmosphere 

 of small— but not small relatively to the wave-length 

 —hollow spherical vesicles reflecting and refracting 

 according to the ordinary- laws for extended media. 

 In a much later paper Rayleigh has given good reason 

 for inferring that at least one-third of the scattered 

 light is diffracted from the molecules of the air them- 

 selves (see also Nature, March 10, 1910, p. 49). 



One of the merits of Exner's discussion of the sub- 

 ject is the care with which he has followed out the 

 application of Rayleigh 's theory in the light of the 

 best recorded observations. These relate to colour, 

 polarisation, extinction coefficients, the general bright- 

 ness and the distribution of brightness of the sky, 

 &c. ; indeed, the whole volume may almost be regarded 

 as an excursus on the Rayleigh theon-. In dealing 

 with the more detailed phenomena of Arago's "posi- 

 tive" and "negative" polarisation, and the neutral 

 points of Arago, Babinet, and Brewster, the author 

 follows Soret in attempting a general explanation in 

 which account is taken of the further action of other 

 particles on the light already once diffracted, and of 

 the form of the limited portion of the atmosphere from 

 which the light reaching any individual eve can be 

 received ; but the conditions are too complicated to 

 admit of quantitative treatment for exact comparison 

 with observation. 



