December 21, 191 1] 



NATURE 



265 



produced on cutting such a section-plate which shows 

 absolutely no trace of a junction-plane in ordinary light or 

 when the Nicols are crossed or parallel, but exhibits 

 different colours in the two halves the moment either 

 Nicol is rotated to even the slightest extent. 



If the plane of junction be oblique, however, the biquartz 

 shows a black band between the two halves when the 

 Nicols are crossed, and a white one when they are parallel, 

 as in the case of a natural biquartz projected on the screen, 

 the obliquity of the junction plane in which is not great. 

 When the junction-plane is of greater obliquity, not only 

 the central black band, but a white one and a spectrum 

 on each side of it, are exhibited, as illustrated on the 

 screen by an example. 



A very instructive case is that of a natural quartz twin- 

 combination, which by its picture on the screen in polarised 

 light, illustrated in Fig. 15, is seen to be composed of one 

 half of left-handed quartz, which polarises in a rich crimson 

 lake with crossed Nicols, and a second half which is made 

 up of alternating right- and left-handed quartz, the strips 

 'i' ing joined obliquely to the plate, so that a black band 

 produced in each case as the central line of a ribbon, 

 iijing flanked by a white band and a spectrum band on 

 ' ach side. 



Such a banded structure of alternately right- and left- 



inded quartz affords an instance on a larger scale of the 

 nely laminated twinning of the two varieties present in 



Fig. 15. — Banded Quartz Twinning. 



inethyst. The case of a section-plate of amethyst is an 



Kceedingiy beautiful one. The marginal parts show 

 [Mjlarisation colours in three sectors, indicating the presence 

 nf right- or left-handed quartz. The alternate sectors, how- 



ver, show a natural violet colour, which is the distinctive 

 ature of most amethysts: hence the name; and in the 

 ntral part of the plate these sectors exhibit the beau- 



ful laminated twinning effect, under crossed Nicols a 



'licate slate-coloured line marking the junction of each 

 l)air of right and left laminae, each lamina itself appearing 

 a^ a thin white band bordered by traces of spectra. On 



';.iiiiii; ill' .inalysing Nicol the laminae exhibit the most 

 iiiiiiiil h m1 - of greys and browns, with delicate linings 

 iii rose and other tints. This central i)art of tho plate, 

 moreover, is unique among quartz cry--tals in ixhihiiint^ a 

 normal uniaxial interference figure in convergent polarised 

 lii;lii, iliai is, with the black cross complete to the centre. 



I luis \\v inlitnate lamellar twinning of the two varieties 

 of quarl/ in anif^thyst results in the production of a crystal 

 whii li sinuilai'S holohedral trigonal symmetry, and is 

 app.'ui ntl\- o])ii(ally inactive, the optical activity of the 

 Ifi-.'. . r-( la- . pail- of variftifs hcint^ mutrallsod by their 

 intinial'' M.iidini^. .Aini't li vsl , tll<'i'cf()r'', i- n'lt n rnso of 

 Irn- iiplit.-il iii:irli\-il\-, liut of nn'i-lianii-.ii^ ' ilni-allv 



proiln.- (1 p,i ml,, mi.m livilN". Il Is thus ai :;! illus- 



■ iiioii on :' i- of ill- still inori' inliniaN' 1. landing 



ni'Mo-, .:; 1 ,iil,niii roscopil' lamina' of llif two 



'^aii'iji-, in 111- il.si.nii.,, I<iio\\n |.i ili. mi^t-, .as 



pseudo-racemic compounds, a large number of which have 

 been studied by Kipping and Pope. In many of these cases 

 the blending of the two varieties by regular intercalation 

 and alternation of the two varieties, one within the other, 

 is -so intimate that the laminae approach the molecular 

 dimensions in thinness. It is easy to see that if the 

 approach be continued until this fascinating region of 

 molecular forces be penetrated — for we know that within 

 four or five molecular diameters these intermolecular forces 

 come info play—chemical allinity will be developed with 

 the production of a molecular compound — a racemic com- 

 pound — of the two varieties, the individuality of each 

 variety being no longer preserved, but a new compound, 

 due to the double molecule, being produced and crystallo- 

 graphically developed according to its own crystalline form. 

 The investigation of the twinning of quartz, as seen in 

 its most beautiful development in amethyst, has thus 

 enabled us to elucidate some of the most interesting and 

 complicated phenomena of optical rotation and of chemical 

 crystallography. ' 



THE VALUE OF BIRDS TO MAN.'' 



WEGETATION is the prime requisite for the perpetuity 

 of all other forms of life upon the earth. The 

 greatest known enemy to vegetation is insect life, while 

 bird life, by virtue of its predominating insect diet, wields 

 a most important balance of power against the ravages 

 of this the chief pest of vegetation. 



The number of insect species is greater by far than that 

 of the species of all other living creatures combined. The 

 voracity of insect life is as astonishing as its power of 

 reproduction. Many caterpillars consume twice their 

 weight in leaves per day, which corresponds to a horse 

 eating daily a ton of hay. 



The development of young birds is so rapid, and the 

 demand upon the vitality of older ones so great, that an 

 enormous amount of food is necessary to sustain the vital 

 processes. Digestion is exceedingly rapid in birds ; and 

 they feed for the most part throughout the day, especially 

 when rearing young. The number of insects daily passed 

 into the insatiable maws of the nestlings during this period 

 almost exceeds belief. But the most valuable services of 

 the adult bird are rendered when it is feeding in winter 

 or early spring, for then 't destroys countless numbers of 

 insects in the embryo state, and thus prevents myriads of 

 depredators from coming forth. Grave and far-reaching 

 results invariably follow the suppression of this perennial 

 regulative influence which is exerted by birds individually 

 everywhere as a check on insect life. 



Forest trees have their natural insect foes, to whirli lh.\ 

 give food and shelter ; and these insects in turn havi- tlnii 

 natural enemies among the birds, to which the tree also 

 gives food and shelter. Birds are not only essential to the 

 well-being of the tree, but the tree is necessary to tlie lifi^ 

 of the bird. It is because of this most delicate adjustment 

 between the tree, the insect, and the bird that Mr. I'lank 

 M. Chapman's statement " that it can be clearlv d' mon- 

 strated that if we should lose our birds we should also losr 

 our forests," must be regarded as profoundly true, (^dl 

 the bird in the orchard an evil if you will. But it is a 

 necessary evil, and the fruit-grower must m.dce up his 

 mind to pay the bird its wages, even thom^li .it lim. s they 

 may seem exorbitant. 



Each season, until hay-making commences, the er.iss 

 offers cover and shelter for the nests of such birds .is tr ■ d 

 on the ground. The fields also provide food for hinK, and 

 for the insects on which birds feed. Wii. re the liinls of 

 the field are undisturbed they tend to hold the ijr.iss 

 insects in check. On the other hand, when llie numliers 

 of birds in th.' field are, for any reason, insulTM i. nt, tli.' 

 insects increas, 



Witlioiii l.'ds gra.ss could not be thrown. Tl'- .l,.\^ ol 

 a sin ~ of beetle, if unchi dl 



the ^i of our meadows, or ' d 



1 The conrhi-Iiiii: !'•. tiirc. wliirti on 

 Of (|ii;iri/, docs not lotul itself to .ibbr.-. 

 Foiirn.ll of the Royal Society of .'\r|s (,l 1 '. i..ii.-, .; vv.ji. i.a., >■ . -i- 



be con.sulled. . . 



3 From a paper read before the Hiilish Ilmpirc Natur.ilil^ .Avociation 

 on Decemlii^r i ty James Ruckl.-ind. 



S8 



