October 14, 1920] 



NATURE 



223 



optical constants of the materials present in the sur- 

 face. The result of his work is published in No. 7 of 

 the Proceedings of the .American Philosophical Society 

 for the current year. Unfortunately, Mr. Wright finds 

 that these constants cannot be satisfactorily found 

 by the method, although there is no difficulty in 

 ascertaining whether the substances seen are crystal- 

 line or not by the rotation of the plane of polarisation 

 of plane polarised incident light. Tests of colour, 

 hardness, and behaviour under the action of solvents 

 still remain the most trustworthy for the determina- 

 tion of the substances exposed at the surface. 



.Among the announcements of forthcoming books 

 just issued by the Oxford University Press is one 

 which should appeal especially to readers of Nature, 

 viz. vol. ii. of "Studies in the History and Method 

 of Science," edited by Dr. C. Singer. The work will 



ontain the following contributions : — Greek Biology 

 and its Relation to the Rise of Modern Biology, Dr. 

 C. Singer ; Medieval .\stronomy. Dr. J. L. E. Dreyer ; 

 Leonardo as .Anatomist, H. Hopstock; Science and 

 Hypothesis, Dr. F. C. S. Schiller ; The .AsclepiadiE 

 and the Priests of .\sclepius, E. T. Withington ; His- 

 tory of .Anatomical Injections, F. J. Cole ; The 

 Scientific Works of Galileo, J. J. Fahie ; Unity in 

 .Modern Scientific Thought, F. S. Marvin ; Four 

 .Armenian Tracts on the Structure of the Human 

 Body, F. C Conybeare; Roger Bacon and the State 



f Science in the Thirteenth Century, R. Steele; -A 

 History of Palaeobotany, Dr. E. N. .A. .Arber; Science 

 and Metaphysics, J. W. Jenkinson ; and .Archimedes' 

 Principle of the Balance and some Criticisms upon it, 

 j. M. Child. The same publishers also promise ".A 

 History of Greek Mathematics," by Sir Thomas 

 Heath, in two volumes. 



Sir Isaac Pitman and Sons, Ltd., have in prc- 

 iration a new series of Technical Primers, the aim 

 f which is to present a sound technical survey of 

 fundamental facts, principles, equipment, and prac- 

 ;ife in volumes covering ultimately all phases and 

 ranches of technology. The earliest volumes will 

 K-al with Continuous-Current Armature Winding, 

 Belts for Power Transmission, Municipal Engineer- 

 ing, Water-Power Engineering, Photographic Tech- 

 nique, Foundrywork, Pattern-Making, Hydro-F.lectric 

 Developments, The Electric Furnace, Small Single- 

 Phase Transformers, Pneumatic Conveying, The Elec- 

 trification of Railways, and The Steam Lorgmotive. 



TiiP. Cambridge Pocket Diary for the academic 

 ar 1920-21 has been issued by the Cambridge Uni- 

 versity Press, price 3.5. In addition to the informa- 

 tion usually found in diaries, a complete list of Uni- 

 Tsity oftirials, dates of examinations and of meetings 

 ' oflficials, and local information, such as Cambridge 

 lophone numbers and the train service to London 

 n'i to Oxford, arc included in this convenient little 



The I'nited States Coast and Geodetic Survey has 



, iiblished a short list (No. log) of its publications 



-.ince January i, 1914, which is supplied free of charge 



'M those interested. The list includes papers in 



•odesy, magnetism, cartography, and hydrography. 



NO. 2659, VOL. T06] 



Our Astronomical Column. 



The Italian Astronomical Society. — This society 

 is commencing the publication of the third series of 

 its Memoirs. Vol. i.. No. i, has just appeared, and 

 contains several papers of interest. The Rev. J. G. 

 Hagen writes on the galaxy and the " Via Nubila " — 

 a name that he suggests for a stream of nebulae, both 

 luminous and dark, having its maximum density in 

 the well-known nebulous region round the galactic 

 North Pole. G. -Armellini contributes a paper on the 

 gravitational potential of the galaxy, and endeavours 

 to explain certain small anomalies in the motion of 

 the planets Saturn and Neptune by stellar perturba- 

 tions. It is, however, fairly clear that on any reason- 

 able assumption of the mass of the stellar system 

 its differential action within the planetary family 

 must be wholly inappreciable. A. Bemporad publishes 

 a series of comparisons of 32 Geminorum with the 

 neighbouring stars { and e Geminorum, made with a 

 wedge photometer between December and March last. 

 It is concluded that the magnitude of 32 Geminorum 

 varies between 6-5 and 72 in a period of 2-43 days; 

 the light-curve is not given, but it is stated to be 

 " of the usual type of short-period variables, with a 

 pronounced secondary maximum." E. Bianchi notes 

 that the minor planet (44) Nysa varied in light with a 

 range of half a magnitude in a period of thirty-six 

 days ; these observations were made in March and 

 .April, 1913. On the other hand, observations in 1920 

 indicate the much shorter period of 3^ hours ; this 

 is presumably an effect of rotation, in which case the 

 monthly variation previously observed needs some 

 other explanation. Other papers deal with the giant 

 and dwarf stars (G. Zappa); measures of the sun's 

 diameter, which appear to show some variation in 

 the value in the course of the sun-spot cycle (A. 

 Prosperi) ; and stellar spectra with an objective prism 

 (G Abetti). 



The Colour of Nebulous Stars. — Sir W. Herschel 

 discovered 130 years ago that certain stars were sur- 

 rounded by nebulous envelopes, and made the correct 

 deduction that the latter were gaseous, not stellar. 

 These objects play an important part in theories of 

 cosmogony, and deserve careful study. The .4stro- 

 physical Journal for July contains a paper by F. H. 

 Scares and E. P. Hubble which establishes the 

 interesting fact that these stars are redder by about 

 one colour-class than would be expected from their 

 spectral type. This was first noted visually by Hubble 

 in one or two cases ; a systematic campaign was 

 therefore planned with the Mount Wilson fxy-'in. 

 reflector, using the method of exposure ratios. .About 

 fifty stars were examined, being roughly half of the 

 known objects of this type. Their spectra are mainly 

 of type B, with a few early .A's, one Oes, and one G^. 

 Check stars were photographed at the same altitwle 

 to determine the plate constants. The stars mainly 

 lie in two groups, and the mean excess of colour- 

 class for these groups is 128 and o-88. In a few 

 cases where the effect does not appear it is stated 

 that the visual aspect suggests that the star is not 

 really in the nebula, but accidentally projected upon it. 



There is a discussion of possible explanations ; the 

 most straightforward is that the red colour is pro- 

 duced by molecular scattering in the nebulous 

 envrlope. Fluorescence in the nebula near the star's 

 surface is also .suggested ; alternatively, that the 

 presence of the envelope may have an influence on the 

 star's constitution. It is noted that if the envelope 

 were composed of particles above n certain size, 

 general, and not differential, absorption would result, 

 so that this may explain the non-occurrence of the 

 phenomenon in a few ca.ses. 



