276 



NATURE 



[November 13, 1919 



The "Daily Telegraph" Victory Atlas of the 

 World. Part i. (London: "Geographia," 

 Ltd., 1919-) Price is. 3d. net. 

 This is the first part of a new atlas to be com- 

 pleted in about forty-eight parts. Each part is to 

 consist of three double-page maps, 20^ in. by 

 26 in. A gazetteer is to complete the work. The 

 first part contains maps of Australia (physical), 

 South-West Spain (political), and Germany (his- 

 torical), besides several inset maps. The colour 

 printing is good and the lettermg particularly 

 legible. The orographical map of Australia is 

 layer coloured, and although it shows some small 

 discrepancies from the recently published official 

 orographical map of the Commonwealth it is an 

 effective and useful sheet. The map of Spain, 

 which we take to be the type of political map of 

 the atlas, would be improved by the omission of 

 the " caterpillar " relief, which is merely mislead- 

 ing and of no value. In this respect the map 

 of Germany is better, for no attempt is made to 

 show relief on it. The changes due to the Peace 

 Treaty are incorporated, but a mistake is made 

 in the area of the Slesvig plebiscite. The atlas 

 promises to be a useful one for general reference 

 purposes. Its low price is much in its favour. 



R. N. R. B. 



The Mica Miner's and Prospector's Guide. By 

 Archibald A. C. Dickson. Pp. viii-t-50. 

 (London: E. and F. N. Spon, Ltd., 1919.) 

 Price 45. 6d. net. 

 The mica industry is indebted to the author of 

 this "Guide" for the current system of mining 

 in Kodarma, the most prolific mica field in the 

 world. His memoirs on that field are well known. 

 Any contribution of his to the literature of the 

 subject is therefore sure of careful consideration. 

 The present booklet, which is high-priced — fifty 

 pages for 45. 6d. — was prepared to help the in- 

 creased output of mica necessary during the war. 

 It contains much valuable information, but does 

 not cover all the ground that might be expected 

 from the title. It consists mainly of descrip- 

 tions of eight of the secondary mines of 

 the Kodarma field and of notes on the 

 mining methods there. It contains little 

 information as to costs and values, and would 

 not explain to a miner who had no previous ex- 

 perience of mica-mining how to estimate the prob- 

 able profit or loss of a newly discovered deposit. 

 The author's main thesis is that mica-mining must 

 be guided by careful geological study, and he 

 insists that all the facts observable during the 

 working of a deposit should be systematically 

 entered on a mine plan. This warning is especially 

 useful with a branch of mining in which so much 

 of the output is from small mines worked by 

 parties of local labourers. Mr. Dickson points out 

 that the mica lenses on the margin of a deposit 

 are apt to be inclined to the shoot, and a miner 

 who was guided only by the facts seen would be 

 diverted from the main body of mica. 

 NO. 261 1, VOL. 104] 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for optnions 

 expressed by his correspondents. Neither can he undertake to 

 return, or to correspond with the writers of, rejected manu- 

 scripts intended for this or any other part of Naturk. iVo 

 notice Is talzen of anonymous communications.] 



Scattering of Light by Resonating Molecules. 



Prof. R. W. Wood (Phil. Mag., vol. xxiii., p. 68t), 

 1912) found that mercury vapour, even at the small 

 density corresponding to atmospheric temperature, 

 when illuminated by the ultra-violet mercury radiation 

 A 2536, re-emits this radiation laterally in considerable 

 intensity. 



Further, Wood and Kimura found after repeated 

 examination ihat this radiation is completely free from 

 polarisation {Phil. Mag., vol. xxxii., p. 329, 1916). 



I have been very much impressed with the contrast 

 between this case, where there is resonance, and the 

 behaviour of gases in general when illuminated by 

 light whicli is not in resonance with the free period 

 of the atoms. In the latter case the laterally emitted 

 light usually approximates to complete polarisation 

 (Proc. Roy. .Soc, A, vol. xcv., p. 155, 1918). What 

 happens as we gradually depart from exact resonance? 



Prof. Wood's experiments were made with the 

 exciting light polarised, and he observed the reson- 

 .ince radiation through the same window bv which 

 the exciting light entered. In this way the light 

 examined comes chiefly from the very first stratum 

 of vapour entered by the beam. This stratum gives 

 by far the most intense emission. 



As, however, the beam advances into the mercury 

 vapour the light in exact resonance is absorbed, being, 

 in part at least, re-emitted. The lateral emission 

 further on is much fainter, and corresponds presum- 

 ably to a less exact resonance. 



It appeared to be of interest to examine this lateral 

 emission from the deeper strata for polarisation. This 

 I have done, and I give here a brief statement of 

 results, leaving the experimental details for later 

 publication. 



After the unpolarised primary beam has traversed 

 0-4 cm. of mercurv vapour at ordinary temperature, 

 the lateral emission shows verv perceptible polarisa- 

 tion, the component intensitv vibrating parallel to the 

 exciting beam having 90 per cent, of the intensity of 

 the perpendicular component. 



.\itcv passing through 2-5 cm., this ratio fell to 

 82 per cent. 



.After 274 cm., the value found was 60 per rent. 



Thus it appears that the scattered light is un- 

 polarised onlv when resonance is very exact. The 

 breadth of the absorption band (reversal) produced 

 by a few millimetres of mercury at atmospheric tem- 

 perature and in vacuo must be extremely small, and 

 probably beyond the range of any but the most power- 

 ful spectroscopes. Yet it is only within this narrow 

 spectral range of the exciting lisht that the scattered 

 light is sensibly unpolarised. When this component 

 is filtered out. and such excitation as remains^ is by 

 light nearer the edges of the exciting line, polarisation 

 becomes conspicuous. 



It need scarcely be said that there is very much 

 more to be done in this direction. Further experi- 

 ments are in progress. Rayj.kigh. 



Imperial College, South Kensington. 

 November i. 



Vertical-pipe Irrigation for Orchards and Market- 

 gardens in Arid Climates. 



In the issue of Nature for September 11, 1919 

 Co. 44). abstracting from the Comptes rendus of the 

 Paris Academy (August 25, 1919, p. 391), mention is 

 made of a new method, proposed by M. Paul Par- 



