April 29, 1922] 



NATURE 



549 



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 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of anonymous communications.'\ 



Discoveries in Tropical Medicine. 



It is a matter for regret that the obituary notice of 

 a well-known medical man should be used to put 

 forward a statement of his share in the progress of 

 knowledge which is misleading. It is necessary to 

 correct such a statement when it is conspicuous and 

 likely to be accepted as truthful. In a brief biography 

 of the late Sir Patrick IManson in the Times of April lo, 

 it is stated that " modern tropical medicine " was 

 born when he suggested that the Filaria sanguinis 

 hominis — discovered some years previously by Dr. 

 Timothy Lewis in persons afflicted with elephantiasis 

 — " is taken from one person to another by mos- 

 quitoes." On the ground that this was the first 

 suggestion as to the carriage of disease-germs by 

 mosquitoes, and was well founded, the chief merit in 

 the later discoveries of the part played by those 

 insects in the transmission of malaria and of yellow 

 fever is attributed by the Times to Sir Patrick 

 Manson. 



This, however, is a misapprehension, the propaga- 

 tion of which must do injustice and falsify history. 

 The fact is that Manson's " suggestion " that the 

 Filaria of elephantiasis is actually carried by mos- 

 quitoes from the blood of one person to that of 

 another remains to this day a "suggestion." It has 

 not been established as a fact. 



Another important misconception enunciated in 

 the same article is that no suggestion as to the mode 

 of entry of the malaria parasite into the blood of 

 human beings was made until Manson, fourteen years 

 after their discovery by Laveran, " suggested " 

 mosquitoes as the carriers. The fact, on the con- 

 trar\^ is that Laveran himself had stated this to be a 

 possible and not improbable mode of transmission, and 

 that the notion was long ago prevalent in India. The 

 man who actually " discovered " the fact of the 

 carriage of malaria germs by a mosquito and the 

 particular species {Anopheles maculipennis) so con- 

 erned, as well as important facts as to the multiplica- 

 ion of the malarial parasite in the gnat's body, is 

 Sir Ronald Ross. 



Finally, the Times states that General Sir David 

 Bruce is a disciple of Sir Patrick Manson. This is a 

 peculiarly unfortunate assertion, for it makes it 

 necessary to state the fact, well known to their col- 

 leagues, that Bruce, so far from being a disciple of 

 Manson, disapproved of his suggestions and of his 

 methods. The man of genius who discovered and 

 finally abolished Malta fever, who by laborious years of 

 w ork in Africa gave us solid and absolutely new know- 

 ledge of the Tsetze fly and Trypanosome diseases, 

 nagana and sleeping-sickness — not to mention his 

 Aar work in conjunction with pupils and colleagues 

 u tetanus and on trench fever — was not in any way, 

 lirectly or indirectly, influenced by or associated 

 ith Sir Patrick Manson. The attempt to associate 

 Llie discoveries of Bruce with the Manson legend is a 

 mere assertion regardless of fact and of the pain which 

 it must cause to the friends of both. 



E. Ray Lankester. 

 April 17. 



Atmospheric Refraction. 



In Nature of August 11, 1921, p. 745, appeared my 

 letter in which I criticised a result stated by Mr. 

 Mallock in Nature of June 9, p. 456 ; and also a 

 further brief letter by Mr. Mallock. Further letters 

 have appeared by. Dr. Ball (January 5, p. 8) and 

 Instr. Commander Baker, R.N. (January 5, p. 8, 

 and January 26, p. 105). In his second letter Mr. 

 Mallock says that " the pressure gradient near the 

 ground, and the density and refractive-index gradients 

 also, decrease linearly at such a rate that if the linear 

 relation continues to hold, the pressure and density 

 would be zero and the refractive index unity at 

 height H." This statement is incorrect as regards 

 the density and refractive-index gradients, except 

 in the special case when the temperature gradient 

 is zero. 



The relation between density and refractive index, 

 Ai-i = K/j, where K is constant ( = 222-16 for sodium 

 Jine D), shows that dfj.jdh = Kdpldh, and so it is only 

 necessary to consider the density gradient. Using 

 suffixes 1, to indicate values at sea level and at the 

 upper limit of the atmosphere, the height H of the 

 homogeneous atmosphere is given by J^ pgdh = 

 Pigiii=pi. Hence pilli = pigi, and by the ordinary 

 mechanical law of equilibrium this is equal to 

 - {dpldh)i, proving Mr. Mallock's statement as 

 ,regards the pressure gradient. 



Now if the absolute temperature near the level 

 considered (sea level) is given by t = ti{i-ah), then 

 p = Ctp = Cti{i-ah)p, which differentiated logarithmic- 

 ally gives for A = o, {dpldh)i= - pi{i-lla)lil. This 

 result agrees with Mr. Mallock's only when a = o, 

 that is, when the temperature gradient is zero. 



The curvature i/o- of a ray inclined at an angle <f> 

 to the vertical is derived from fir sin <p = constant, dxp = 

 d(l> + sin (pdsir, whereby ijar =dflds= -K sin (pdpjdh^ 

 (i-Ha)sin?i.K/)/H = (i-Ha)sin9i(/x-i)/H, a result 

 applicable to any point of the ray if H is understood 

 to mean the height of the homogeneous atmosphere, 

 of density p at the point, above the point. Here again 

 agreement with Mr. Mallock is obtained if a = o 

 and <^ = 9o°, i.e. for a horizontal ray. 



Introducing the temperature gradient ^^t^a, we 

 see that ila = Cl3lgi = 2g-28^gJg. Also (/a- i)/H varies 

 as Bgjt^, B being the barometer reduced reading. 

 Hence 



- = 3-665x10-5--^ . 

 <7 -^ ^ 760 g,^ 



{i+at)-'{ 



I - 29-2i 



g 



I sin </>, 



NO. 2739, VOL. 109] 



where a = 1/273, ^' is fall of temperature t per metre, 

 and <r is in kilometres. No mention has been made of 

 humidity for the reason that its action is chiefly to 

 modify the temperature gradient, which has been 

 allowed for. 



If we multiply ija by the earth's mean radius, 

 6371 km., we obtain 2k, according to the Indian 

 definition of coefficient of refraction k ; or k, according 

 to the continental definition. The result is to change 

 the num^ical factor in i/a into 0-2335. This agrees 

 well with Jordan's formula quoted by Dr. Ball, 

 which I had not previously seen and which, I believe, 

 has never been used by surveyors. For B = 760 mm. 

 and latitude 45° we can write 



<r = 27285 (i-f-a/)2(i-29-28/i')"^ cosec <(> kilometres. 



The following table gives values of the radius of 

 curvature <t in miles of a horizontal ray at levol 

 B = 76omm., and the coefficient of refraction as 

 defined in India. 



T 2 



