November io, 192 i] 



NATURE 



JO/ 



has a duration of four thousand years ; men are fond 

 of recording wonders ; yet never has a useful and 

 persistent human mutation been recorded. 



Judging, then, from crucial examples, (i) natural 

 varieties have evolved by way of fluctuations, but 

 artificial varieties in great measure by mutations ; 

 (2) there is no Mendelian segregation, but only Men- 

 delian reproduction ; (3) blending is universal ; and 

 (4) apart from variations (including the results of 

 blending), like, always and necessarily, begets like 

 when parent and child develop under like conditions 

 of nurture. 



Surely it is evident that if we use precise language 

 (as Darwin tried to do), and bring all the available 

 evidence into court by means of crucial examples (as 

 Darwin did), the dust which \eo-Lamarckians, Neo- 

 Darwinians, and the rest of the sects have flung into 

 our eye^ will be washed awav, and our verv great 

 man will come into his kingdom again. 



G. Archpall Reid. 



Methods of improving Visibility. 



The observations of Prof. C. V. Raman (Natlre, 

 October 20, p. 242) on a method of improving the 

 visibility of distant objects by the elimination of 

 reflected light by means of a S'icol's prism placed in 

 the eye-piece of a telescope are certainly interesting, 

 but at the same time they are thoroughlv well known, 

 and the idea of increasing the visibilitv bv the 

 elimination of polarised light has received a great deal 

 of attention lately. During the war a considerable 

 amount of experimental work was carried out bv 

 the Admiralt}- in connection with fog-penetration and 

 the beamless searchlight, both of which dealt with 

 polarisation phenomena. It was, however, found 

 advisable to substitute for the Xicol's prism as used 

 bv Prof. C. V. Raman a few plates of plain glass 

 placed obliouelv at a suitable angle across the axis 

 of the particular instrument, since it is extremelv 

 difficult to mnke really large Nicol's prisms, in 

 addition to which the actual absorption by Nicol's 

 prisms is rather excessive. The matter was also 

 taken uo bv the late Sir William Crookes, and formed 

 the basis for lenses cut from quartz cr\-stals at right 

 angles to the principal axis, thus utilising the rotary- 

 polarising effect of this material. I h^ve latelv been 

 using tourmaline for obtaining a similar effect, and 

 T have found that a verv thin plate of tourmaline 

 cemented as a semi-lens on to an ordinary pair of 

 spectacles is best for this purtx>se. 



Tourmaline is a mineral that has found a great 

 deal of application latelv. and oarticularlv during the 

 ■war in connf^ction with the piezo effect, in the ap- 

 paratus used for the detection of submarines and 

 submarine-sounding, and is, in conseauence, fairlv 

 abundant. For this purpose the plates of tourmaline 

 are cut perpendicular to the vertical axis, but the 

 maximum polarisinf? effect is obtained from plates cut 

 parallel to this axis. Thin plates of this material, 

 when so cut and fixed into the spectacles as men- 

 tioned, give an effect the benefits of which can only 

 be realised when out to the actual test. Thus, for 

 ■example, when fishing, the injurious glare from the 

 water is ^ntirelv eliminated. At the same time the 

 eve is able to penetrate the water to considerable 

 ■depths. This fact will, of course, b^ appreciated bv 

 those engaged in the studv of pond-life. 



A further application is the manufacture of spec- 

 tacles for invalids and others residing at the seaside, 

 whereby, again, the glare of the water is almost 

 entirely eliminated and the delightful tone of the 

 tourmaline is ver\- restful to the eye. In photographv 

 a further a|)plication consists in using a sheet of 

 NO. 2715, VOL. 108] 



tourmaline as a light-filter, whereby reflection, and in 

 particular that from shining objects, is largely 

 eliminated ; and while it is not possible to take 

 photographs of water directly facing the sun, many 

 pictures which are otherwise impossible can be taken 

 by means of this screen. 



.\ final application, and one w-ith which my experi- 

 ments originated, was the examination of photo- 

 micrographs, which, owing to their delicate nature, 

 were of necessity- mounted below a sheet of glass. 

 The continued observation of these objects was found 

 to be a verv tedious process owing to the brilliant 

 light required and the consequent reflection from the 

 glass. This reflection was again eliminated by the 

 tourmaline. The plates of tourmaline must, of 

 course, be so mounted that the vertical axis is placed 

 vertically in the spectacles. 



These applications have already been provisionally 

 protected at the Patent Office, but up to the present 

 I have found no firm that will take up the manufac- 

 ture of such glasses. I am confident, however, that 

 a ver\- useful industry awaits the firm with the 

 necessarv enterprise. A. G. Lowndes, 



Marlborough College, Wilts, October 28. 



Prof. C. V. Raman's suggestion (N.ature, October 

 20, p. 242) for improving visibility at sea was put 

 forward by me some years ago in " Elementary' Sea- 

 manship " (Griffin and Co.), and is useful not only 

 for objects above the surface, but for those — lik* 

 coral-reefs — some little distance below the surface. In 

 the last instance, when there is a slight ripple on the 

 surface of the water it is often difficult to detect them 

 even when navigating with the sun astern. The use 

 of the \icol prism clears up in a great measure this 

 difficulty. David Wilson-Barker. 



Flimwell. October 28. 



Penial and Genital Sets of Lumbricus terrestris, L., 

 Miill. 



On p. 172 of Prof. O'Donoghue's " \x\ Introduction 

 to Zoology for Medical Students," reviewed in N.ature 

 of August II, it is stated that in Lumbricus herculeus 

 (the name is a synonym of L. terrestris) "in the 

 fifteenth segment the two pairs of ventral setae lying 

 close to the male external aperture are modified to 

 form the penial setae." In Bourne's "An Introduc- 

 tion to the Study of the Comparative .Anatomy of 

 .\nimals," vol. 2, pp. 19-20 of the fifth edition, 1912, 

 it is said that in the same worm " the chaetae of the 

 clitellar region differ from those of the rest of the 

 body, being finer and nearly straight, with hnoke'^ 

 inner ends. There is also a pair of modified chaetae 

 in somite 15." Borradaile, in "A Manual of Elemen- 

 tary Zoology," p. 217 of the third edition, 1920, says. 

 "The ventral setae of the clitellum, of the 26th and 

 of the loth to the 15th segments are straighter and 

 more slender than those of other segments, which are 

 stout and somewhat hooked. The modification is in 

 connection with the use of the setae of the 26th seg- 

 ment during coition, and of the other straight setae 

 during the formation of the cocoon in which the eggs 

 are laid." Parker and Haswell, in ".X Textbook of 

 Zoology," p. 455 of the second edition, 19 10. state 

 that "the setae in the clitellum, and those in the 

 neighbourhood of the genital apertures, are much 

 slenderer than t'ne rest." 



Svstematic writers, however (to whom modifications 

 in the form of the setae are of importance as furnish- 

 ing specific characters), do not appear to have recog- 

 nised the presence of penial setae in segment 15 of 

 Lumbricus terrestris. Thus Beddard, in "A Mono- 



