May 27, 1922] 



NATURE 



68 



which can be shovm with certainty on the galvano- 

 meter. Accuracy will depend, among other things, 

 on the accuracy of the weights and balancing resist- 

 ances employed, while the definition is decided by 

 the workmanship and design of the instrument 

 alone. 



In optical instruments, resolving power is judged 

 by the smallest angular or linear quantities which 

 can be distinguished by their means, and definition by 

 the relation which these quantities have to the size 

 of the field over which they are distinguishable. If 

 a telescope, for instance, can distinguish seconds of 

 arc, its resolving power is 206,000 nearly, but if the 

 field over which this resolution extends is only half 

 a degree, the definition would be 1800 {i.e. 

 206,000/57 X 2). 



Although the performance of any combination of 

 real lenses depends on design and workmanship, it 

 is not difficult to find the limits beyond which even 

 perfection in both does nothing to increase resolving 

 power. 



The function of a perfect lens is to change the 

 radius of curvature of a spherical wave surface. 

 Let D (Fig. 4) be the diameter 

 ^ of the lens where the change is 

 effected, / the new radius of curva- 

 ture, and the geometrical focus. 

 From every part of the wave surface 

 at D partial waves may be sup- 

 » posed to spread, all of which will 

 reach in the same phase. Confining 

 the attention for the moment to 

 -^-Q those rays which start from the 



Fig. 4. opposite ends of a diameter of the lens, 



the partial waves from either end will 

 be in opposite phases at a distance a, in the focal 

 plane, from if 2a sin al2. = \l2, where a is the angle 

 subtended at by D, and X the wave-length of the 

 light. Also, since sin a/2 = D/2/, a//= \/2D and a =f\/2'D. 

 Thus \/2D and /X/2D are the least angular and linear 

 distances from the geometrical focus at which a total 

 absence of light can be found. 



If the partial waves are received from the whole 

 marginal annulus of the lens, in place of those from 

 the extremities of a diameter, the value of Ui is 

 sUghtly increased, and the image about of a distant 

 point of Ught is a bright circular area surrounded by 

 a series of rings. (The rings are caused by recurring 

 coincidence of the phases of the partial waves at 

 certain distances frOm greater than a.) The bright 

 centre and rings are identical with those seen in the 

 well-known experiment in which a bright point 

 appears in the centre of the shadow of a disc, illumin- 

 ated by a bright distant source. 



If the whole area of the lens is employed the 

 diameter of the bright centre is further increased, 

 but the intensity of the rings is much reduced. This 

 case, as it applies to telescopes, has been considered 

 by Airey in a paper on " The Intensity of Light 

 in the Neighbourhood of a Caustic," pubUshed about 

 the middle of the last century. 



In whatever manner, however, the lens is used, 

 whether with a central stop, or with its whole area 

 uncovered, a// and a are the least angles and distances 

 which separate the geometrical focus from the truly 

 dark boundary of the image, although, owing to the 

 rather rapid diminution of intensity from the centre 

 to the circumference of the image, it is possible to 

 recognise angles smaller than X/2D (this applies to 

 telescopes in which a is always small), or distances 

 less than X/2 by the use of microscopic objectives 

 of large angular aperture, where sin a/2 approaches 

 unity. 



The appearances presented at the focus when two 



NO. 2743, VOL. 109] 



objects in such close proximity are examined can 

 scarcely be described as the images of the objects, but 

 rather as interference phenomena which require 

 interpretation. 



A. Mallock. 

 9 Baring Crescent, Exeter. 



Discoveries in Tropical Medicine. 



In Nature of April 29, p. 549, Sir E. Ray Lan- 

 kester criticises an obituary notice on Sir Patrick 

 Manson which appeared in the Times of April 10. 

 The statement chiefly objected to is that " modem 

 tropical medicine was bom the day that Manson 

 discovered the part played by the mosquito in the 

 transmission of Filaria sanguinis hominis." But all 

 parasitologists know that it was Manson who, forty- 

 four years ago, proved by experiment the part 

 played by mosquitoes in the propagation of filariasis. 

 This discovery is not only the pride of tropical 

 medicine, but the very breath of modern medicine 

 and one of the most glorious achievements of British 

 science. 



Strange to say, Sir Ray Lankester to-day repeats, 

 almost word for word, a mistake he published twenty 

 years ago in the Times. He says : " The fact is that 

 Manson 's ' suggestion ' that the Filaria of elephantiasis 

 \_sic\ is actually carried by mosquitoes from the blood 

 of one person to that of another remains to this day 

 a ' suggestion.' It has not been estabUshed as a 

 fact." 



It is surprising that a naturalist of repute, who 

 writes frequently on matters medical, should make 

 this mistake. Surely Sir Ray Lankester must have 

 come across books on parasitology written within 

 the last twenty or thirty years. How can he say 

 that the agency of the mosquito, in the dissemination 

 of filariasis, has not yet been established ? Was it 

 not surmised by the Chinese ages ago ? Was it 

 not suggested by Bancroft of Brisbane (Queensland) 

 in 1877 ? Was it not independently and experiment- 

 ally proved by Manson, in China, that very same 

 year ? Only one thing remained uncertain for some 

 years, and that was the actual method by means of 

 which the young filariae, after reaching a certain 

 stage of development within the body of their insect 

 host, left the mosquito to invade man ; but this was 

 fully established and admirably demonstrated between 

 1899 and 1900 by Manson, Low, Bancroft, Annet, 

 and Dutton, James, Nod, Grassi, myself, and others. 

 Dr. Low's beautiful celloidin sections of infected 

 mosquitoes showing the worms, either quiescent 

 between the massive thoracic muscles of the insect, 

 or actively migrating to the latter's mOuth parts and 

 fixed while passing beneath the cephalic ganglia, 

 gliding down within the labium, or escaping through 

 a rent between the labella, at the distal extremity 

 of the labium, have been exhibited repeatedly, not 

 only at the Royal Society and elsewhere in this 

 country, but also, on several occasions, in France, 

 Italy, and Belgium. Moreover, they have been 

 photographed and even reproduced in colour by skilled 

 artists, and both photographs and drawings have 

 appeared in books on tropical medicine and para- 

 sitology pubUshed since 1900. 



Sir Ray Lankester refers to two other vexing 

 questions, namely, the discovery of the part played by 

 mosquitoes in the propagation of the intermittent 

 fevers and that of tsetse flies in the transmission of 

 certain trypanosome diseases of animals and man 

 such as nagana and sleeping sickness. With regard 



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