54 



NATURE 



[October 7, 1915 



America, C. Z. Southard, illustrated; The Rambles 

 of a Canadian Naturalist, S. T. Wood, illustrated 

 (J. M. Dent and Sons, Ltd.); Bramble-Bees, J. H. 

 Fabre (Hodder and Stoughton) ; Bird Behaviour, F. 

 Finn; Insect Artisans and their Work, E. Step 

 (Hutchinson and Co.); The Book of the Wilderness 

 and Jungle, F. G. Aflalo ; Wild Animals of Yesterday 

 and To-Day, F. Finn; The Common Beetles of our 

 Countryside, W. E. Sharp (S. W. Partridge and Co.) ; 

 Wonders of Animal Life, W. S. Berridge (Simpkin 

 and Co., Ltd.). In Geography and Travel— By Forest 

 Ways in New Zealand, F. A. Roberts, illustrated 

 (Heath, Cranton and Co., Ltd.); Java: Past and Pre- 

 sent, D. Campbell, 2 vols., illustrated (Heinemann) ; 

 How to Read Ordnance Maps, J. F. Unstead; Philip's 

 Human Geographies, H. Fairgrieve and E. Young 

 (G. Philip and Son, Ltd.); With Scott: the Silver 

 Lining, T. G. Taylor, illustrated (Smith, Elder and 

 Co.). In GeoZogy— Earth-Lays : Geological and 

 other Moods, C. Tolly (J. M. Dent and Sons, Ltd.). 

 In Mathematical and Physical Science — Text-Book of 

 Navigation and Nautical Astronomy, Capt. A. P. W. 

 Williamson, new edition, illustrated (John Hogg). In 

 Medical 5cie»ce— Mosquito Control in Panama : the 

 Eradication of Malaria and Yellow Fever in Cuba and 

 Panama, J. A. Le Prince and A. J. Orenstein (G. P. 

 Putnam's Sons). In Tec/inoZo^y— Handbook of Tech- 

 nical Instruction for Wireless Telegraphists, J. C. 

 Hawkhead and H. M. Dowsett, new edition, illus- 

 trated (The Wireless Press, Ltd.). In Miscellaneous 

 —The Limitations of Science, Prof. L. T. More (Con- 

 stable and Co., Ltd.) ; Social Progress and the Darwinian 

 Theory, G. W. Nasmyth; Darwin and the Humani- 

 ties, Prof. J. Mark Baldwin (G. P. Putnam's Sons). 



OUR ASTRONOMICAL COLUMN. 



Measures of Jupiter. — Numerous visual and photo- 

 graphic measures of the diameter of this planet have 

 been made by the Rev. Father S. Chevalier, S.J., of 

 the Z6-Se Observatory, during 19 12-13 {Mem. Soc. 

 Spett. Ital, July, 1915). The Gautier micrometer 

 used in the visual work provides for the measures 

 being made by bringing two strips of platinum tan- 

 gential to the limbs of the planet, either (i) internally, 

 i.e. with both strips seen in front of the disc, or (ii) 

 externally, both strips beyond the disc. The two sets 

 of measures thus obtained were of closely similar 

 accuracy. In the photographic work very many 

 separate exposures were made on each plate, affording 

 material for a considerable body of measures. The 

 exposures were graded in seven groups according to 

 intensity, and measures on only four of these sets 

 were used in taking the means. The discarded groups 

 were those in which the images were considered either 

 too small through under-exposure or enlarged by ir- 

 radiation. The mean results may be summarised as 

 follows : — 



Polar diam. Equat. dam. 



Visual measures ... 35-37" 38-16" 



Photographic measures 35-20" 37'99" 



The value of the aplatissement derived from both sets 

 is practically identical, its reciprocal being 13-6, indi- 

 cating a decidedly greater flattening than hitherto 

 ascribed to this planet. Thus in one of the most 

 important memoirs recently published (Lau, Ast. 

 Nach., No. 4673) the diameters are given as 35-4" and 

 376", yielding 17 as the reciprocal of the ellipticlty. 



NO. 2397, VOL. 96] 



The Rotation Period of Certain Jovian MARKi 

 INGS. — Jupiter is now well situated for observation, 

 and Mr. W. F. Denning, in the Journal of the Royai 

 Astronomical Society of Canada (September), indi- 

 cates some directions in which useful work may be 

 done. The subject of the paper is the rotation period 

 of the great red spot and of the hollow in the southern 

 equatorial belt. The yearly values of the rotation 

 period are tabulated for the eighty-four years, 183 1- 

 19 14 (inclusive). Waiving the debatable point in- 

 volved in the formation of this sequence, the period 

 has varied between gh. 55m. 33-3s. (1831) and 

 gh. 55m. 41-7S. (1898). Special attention is directed to 

 the very marked acceleration since 19 10. The mean 

 period of rotation for about 74,170 rotations comes out 

 at gh. 55m. 36-9S. After such a weighty determina- 

 tion one hesitates to mention a value based on twenty- 

 nine rotations, but M. Chevalier's recent photographic 

 measures are worthy of note (Mem. Spett. Ital., vol. 

 iv., September 2, p. 113). The point measured was 

 the western extremity of the south tropical disturb- 

 ance, and the period deduced gh. 55m. 10-4S. ±o-7s., 

 a faster movement than Mr. Denning assigns to the 

 dusky south tropical disturbance (qh. 55m. 20s. ap- 

 prox.). The question of co-ordinating the visual and 

 photographic observations would no doubt in happier 

 times have led to extensive polemics. 



R. CoRON/E BoreAlis.— This irregular variable has 

 been manifesting light-changes during the past 

 summer (see this column, August 19). We now learn 

 from the Monthly Register of the Society of Practical 

 Astronomy that in Bulletin No. 585 Harvard College 

 Observatory, August 14, 1915, Prof. E. C. Pickering 

 states that attention had been directed to its diminution 

 in brightness by Mr. Eaton, and Mr. Bancroft found 

 its magnitude 6-8 on July 24 and 6-9 on the 25th ; 

 whilst at Harvard, on August 13, its magnitude was 

 only 7-7 (Mr. Campbell). 



A lo-iNCH Diffraction Grating. — A short paper by 

 Prof. A. A. Michelson on the ruling and performance 

 of a giant diffraction grating {Proc. Amer. Phil. Soc, 

 No. 217, p. 137) affords most inspiring reading. 

 Rowland's gratings accompUshed so much, yet the 

 interferometer, the echelon, and the plane parallel 

 plate brought such undreamed-of resolving powers into 

 practical employ that it might well have been 

 imagined that the grating had had its day. Michelson, 

 the pioneer of the newer methods, realised early their 

 disadvantages, and twelve years ago set about con- 

 structing a perfect screw 20 ins. in length, with the 

 hope of some day ruling a grating 14 ins. long. 

 Mechanicians will delight in the ingenious devices 

 which have brought realisation within reach, and even 

 encourage the undertaking of a 20-in. grating. Thus 

 for correcting the screw an interferometer method was 

 adapted enabling measurement to within ixio-^ of 

 an inch. An auto-collimating method of straighten- 

 ing the guiding ways ensured rectilinear motions 

 within one second of arc. The grating carriage was 

 nearly floated in mercury. Longitudinal movement 

 of the screw was geometrically eliminated by allow- 

 ing its spherically rounded end to bear against an 

 optically plane surface of diamond fitted with adjust- 

 ments to secure normality to the axis of the screw. 

 Periodic errors could be corrected by using a worm- 

 wheel to rotate the screw, and those of higher order 

 were eliminated by a mysterious "correcting device." 

 So long ago as 19 10 a 6i-in. grating had been pro- 

 duced and was employed by Messrs. Gale and Lemon 

 in a research demanding resolving powers rivalling 

 those of interference spectroscopes ; now a ruled 

 surface 9I in. x 2^ in., having 11,700 lines per inch, 

 has been achieved and tested in a spectrograph of the 

 Littrow form with an 8-in. 20-ft. Brashear lens. The 



