64 



NATURE 



[March 22, 1917 



The fringes appear to be due to the interference 

 between two portions of light arising out of- the 

 single beam incident on the grating, one of them 

 being reflected at the silvered surface and then 

 diffracted out by the gi-ating element which it meets, 

 and the other being diffracted into the air space by 

 the same grating elemeijt and then reflected out by 

 the silvered surface. The path difference is equal to 



2/ (cos X - cos ^) 



where and X are the angles of incidence and 

 emergence respectively, and t the thickness of the 

 air film. Therefore the condition for interference is 



^4 1 V J.\ 2K+Ix 



2/ (cos X-cos a))= — A 



2 



(i; 



where K is an integer. 



But since we are using the grating, and X are 

 connected bv the relation 



<^/(sin ^-sin X) = n X 



(2) 



where d is the grating interval and n the order of the 

 spectrum. 



Hence, dividing (i) by (2), we have 



tan 



<^ + X (2K+i)^ 

 2 ~ 4/« 



In this equation (0 + X) is the angle which the direc- 

 tion of any particular dark fringe makes with the 

 incident light, i.e with the axis of the collimator. 

 Since (0 + X) is determined once for all independent 

 of the position of the grating, the absolute fixity of 

 the fringes is accounted, for. A detailed paper on 

 this subject was laid before the session of the Indian 

 Science Congress held at Bangalore in January, 19 17. 



C. K. Venkata Row. 

 6 Singarachari Street, Triplicane, Madras, 

 S. India, February 10. 



Mountain Sickness. 



The reference in the Note^. columns of Nature of 

 January 25 (p. 415) to the physical failure experienced 

 in mountain-climbing at high altitudes sent me to 

 the very instructive (and suggestive) article by Dr. 

 A. M. Kellas in the Geographical Journal. And the 

 great interest now attached to one of the inevitable 

 problems of the immediate future gave me to think 

 that a few suoplementary notes might be of sufficient 

 interest for publication. The "mountain sickness" 

 which forms the association that specially interests 

 the physiologist and the physician was impressivelv 

 brought under the notice of the latter in the "fall'" 

 of the fifteenth century; when the gold-thirst of the 

 ruthless Spanish invader of the western Eldorado made 

 him familiar with its symptoms directlv after reaching 

 the very elevated backbone of the southern section of 

 the New World. The oldest special description that 

 appeared in print would seem to have been that of 

 Da Costa; and the verv human appetite for novelty 

 proceeded very soon to make the '' mal de montagnes'" 

 a phrase-name as familiar to Western Europe as that 

 of the mal francais— so very unhappily — rapidly came 

 to be. The syndrome was referred to in the various 

 linguistic territories bordering the giant Cordilleras 

 as : Soroche, mareo des Cordilleras, asthma des nion- 

 tagnes, etc. And the native orophvlactic, on the 

 colossal slopes and towering cliffs of the Cordilleras 

 of Peru, was slow and continuous mastication of 

 prepared pellets of the dried juices of Erythroxylon 

 coca — the original version of American " gum- 

 chewing." The phenomena came in time to receive 

 definite scientific discussion, notably at the hands of 

 Tvn 417 VOT nol 



Bouguier (" Voyage en Peru ") in 1745, and Corw 

 damine (of Peruvian bark fame) in 1751. 



As world-wide scientific mountaineering developed — 

 along lines of modern evolution — a number of French 

 and German observers came to depict in turn their 

 personal experiences of the symptom-group : Saussure,. 

 Clissold, Barry, Rohrdoff, Zumstein, Lepileur, Mar- 

 tins, and Bravais — according to their several personal 

 experiences on Mont Blanc; Humboldt, Boussingault, 

 and Hall — on the upper reaches of Chimborazo. As 

 might be readily anticipated, in a personal experience 

 in which individual constitution and previous training 

 count for so much, we are told by the illustrious Hum- 

 boldt that: " Ces phdnomenes sont trfes-dissemblables 

 suivant I'age, la constitution, la finesse de la peau, 

 les efforts ant^rieurs, les forces musculaires," etc. It 

 very obviouslv corresponds in great, though not ■ ex- 

 clusive, measure to the " incommodites " of the 

 balloon ascent of Biot and Gay-Lussac, on "le 6 

 fructidor, an XII " (August 24, 1804) — greatly ex- 

 aggerated, of course, and developing at a lower alti- 

 tude, from the very laborious muscular exertion of 

 mountain-climbing. The latter rivals, as a factor in 

 physiological derangement, the suddenness of change 

 of environment in a balloon ascent, which bars off all 

 chance of the gradual adaptation which would be so 

 very necessary for functional adjustment. 



John Knott. 



Dublin, March 2. 



BORNEO AND ITS INHABITANTS.^ 



AS explained in the author's unfinished intro- 

 duction, this book is a somewhat discon- 

 nected account of the natural history of Borneo, 

 compiled from notes while he was in charge of 

 the Rajah of Sarawak's museum at Kuching". 



The first chapter deals with the mammals, and, 

 as might be expected, considerable space is de- 

 voted to the orang-outang, or, as the author prefers 

 to call it, the "Maias," this being its correct Malay 

 name. It is satisfactory to learn that this inter- 

 esting representative of the human family is still 

 abundant, though local, in Sarawak. Though the 

 fauna of Borneo is lacking in many of the larger 

 mammals that appeal to the sports<T»an, it has at 

 least its full share of remarkable forms among 

 the smaller species. Many interesting details are 

 given of that extraordinary little lemur, the 

 Tarsier, Tarsius spectrujyr, and of the so-called 

 flying lemur, Galeopithecus volans, that puzzle 

 for systematists which has now the distinction of 

 an Order to itself. The remarkable colour rela- 

 tionships between the squirrels of the island and 

 certain unpalatable tree-shrews of the genus 

 Tupaia are discussed at length. The relations 

 of nalm civets with coffee are at first sight far 

 from obvious, but those of" our readers who obtain 

 their coffee from Borneo and are curious as to 

 the previous history of the best quality berries 

 should consult p. 33 ! It is of peculiar interest 

 to find the mouse-deer taking the place in the 

 native folk-lore of " Brer Rabbit," the latter itself 

 being a direct descendant of the hare which alw^ays 

 figures as the cunning hero in equatorial Africa. 



The second, third, and fourth chapters are de- 



1 " A Naturalist in Borneo." By the la'c Robert W. C. Shelford. 

 Eci'ted, with a Biographical Introduetion, by Prof. E. B. Poulton. Pp. 

 .\xvii+ 331 + xxxi: plates. (London: T. Fisher Unwin, Ltd, 1916.) Price 

 15^. net. 



