November 14. 1:918 



NATURE 



205 



From a variety of sources th< author hi 

 d .1 considerable amount of information into 

 , ,in<l the laj reader may rely upon 

 finding sufficient information for his purpose con- 

 ng our indigenous medicinal and poisonous 

 plants. That inaccuracies occur here and there 

 must be admitted ; th< » due to in- 



sufficient verification on the part of the author, 

 and their present e i pi ising « hen one 



considers licting statements 



that' have atly made on the subject. 



n be called for, these might 

 oided by submitting the proofs to an expert 

 ion, and blemishes thus be removed 

 from a useful little work. 



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 

 '.he writers of. rejected manuscripts intended for 

 this or any other pail of NATURE. A't> notice is 

 taken of anonymous communications.] 



The Colours of the Stria; in Mica. 



On examining even thi mi split and trans- 



parei mica by diffu d light, a few 



fine hair-like and rather irregular lines may generally 



een running along the surface. We have found 



that these lit i show some ver\ interesting 



- ivhen mica is examined in a Topler "Schlieren" 



iratus. The sheet as a whole, being optically 



remains invisible, but the striae shine out as 



brilliant and vividly coloured ' . thi colout 



; different fur diffi reti I < hanging in a 



rkable manner as the inclination "I the mica 



relath : x light in the appat att 



is altered. For it tal ini idem e 



may appear crimson and. as the mica is rotated about 



-.is in its ov sivi l\ purple, 



green, yellowish-j scarl t-red, 



. anil red. 



■ion is In ie. id in detail by 



of us (P. X. Ghosh), but as in its general nature 

 there appears to hi little doubt. The stria? are lines 

 at which the thickness of the mica changes in a dis- 

 "iiious manner, ami the luminosity is due to thi 

 radiation from the discontinuity acting as a laminar 

 diffracting boundary. For an} particular wave-length 

 tin- radiation is zero if ill' retardation of the wave- 

 front on either side of the discontinuity differs In 1 an 

 nulriple of half a wave-length, and is approxi- 

 h a maximum if the difference is an odd multiple 

 -length. The detailed mathematical 

 investigation would follow the general lines indicated 

 h\ Lord Rayleigh in his theon ol the Foucault "knife- 

 iPhil. Mag., February, no;). 



C. V. Raman. 

 P. N. Ghosh. 

 210 Bow-bazar S a, India, 



5. 



Probable the stria-, regarded b\ the authors as 



boundaries between regions different thick- 



m sses, are the same lines ^ , an be seen by reflections 



light, .1- described in no note on "Regularity 



\i teal Crysta ' ' vol. xix., 



D btless the I hod show s 



them in a more striking manner, and, i 

 colour effects are novel, so far as I knov . and worth} 

 of a closer examination. RAYLE1 



NO. 2559, VOT.. I02] 



SELF-CONTAINED M I \ /-. RESCUE 

 APP IB ITUS. 



MOST people are now more or less familiar 

 with the development of the Army respi- 

 ratOI from its crude form of a cloth pad to the 

 scientific and efficient 'bo ipirator " used 



to-day. This is just one example of the many 

 applications and developments ol science during 

 the past lew \i;irs. In milling wor.k the need for 

 the construction of apparatus on scientific lines 

 is being more and more realised, and this is espe- 

 cially so in the case of mine rescue apparatus. 

 When these are employed, whether for actual 

 saving of life, for recovery work after some 

 serious explosion, in dealing' with mine fires, or 

 for any other use in an irrespirable atmosphere, it 

 is imperative that the apparatus should be so con- 

 structed that the wearer may absolutely rely upon 

 it to last for the period and work required. In 

 the past, unfortunately, too many different types 

 pparatus have been put on the market without 

 undergoing' a thorough and scientific testing", and 

 as a consequence in several cases their use has 

 been attended with fatal rest 



The "box respirator" is designed to withdraw, 

 in render innocuous, small quantities of highly 

 toxic gases or vapours, thus leaving- the air for the 

 wearer to breathe practically harmless. Certain 

 gases are, however, not readily absorbed by the 

 ordinary form of Army respirator, and of these 

 carbon monoxide is notable. The highly toxic 

 action of small quantities of this gas mixed with 

 air renders the use of an apparatus of the type 

 of a self-contained mine rescue apparatus 

 essential, and for certain classes of work at the 

 Front, where dangerous quantities of carbon 

 monoxide are met with, such apparatus has been 

 largely employed. 



The recent report of the Mine Rescue Appara- 

 tus Research Committee 1 should prove of interest, 

 therefore, not only to the mining community, but 

 also to many members of his Majesty's Forces. 

 In May, 1917, the Advisory Council for Scientific 

 and Industrial Research appointed Mr. W. Walker 

 (Acting Chief Inspector o\' Mines), Dr. H. 

 Briggs, and Dr. J. S. Haldane as a Committee 

 "to inquire into the types of breathing apparatus 

 used in coal mines, and by experiment to deter- 

 mine the advantages, limitations, and defects of 

 the special types of apparatus, what improvements 

 in them are possible, whether it is advisable that 

 the types used in mines should be standardised, 

 and to collect evidence bearing on these points.*' 



Recent advance in our knowledge of the physio- 

 log v of breathing, largely due to the work of Dr. 

 Haldane, and the hitter's practical tests on various 

 types of mine rescue apparatus at Doncaster 

 during the past few years, together with those 

 il out by Dr. Briggs (for the Research Com- 

 mittee) at Edinburgh, have given the Committee- a 

 nidation upon which to build its report. 



' First Report of the Mine Rescue Apparatus Research Committee. 

 hed for the Department of Scientific and [ndustri 

 H.M. Stationery Office.) Price a. orf. The illustrations ■-. hich accompany 

 this article are reproduced from the Report by permission of ttie Controller 

 -itionery Office. 



