258 



NA TURE 



[July II, 1889 



adapted for making such prism=;, either with wide or narrow 

 angles between the respective planes of polarizatian in the two 

 parts of the visible field. Two such twin-prism?, one with go", 

 the other with 2h°, between the prisms, are here on the table. 

 In the second place, I have essayed a polarimeter, an example of 

 which is before you, in which an arrangement of twin-mirrors 

 (each set at the polarizing angle, but slightly inclined to one 

 another) is made to yield a half shadow effect. 



Before I leave the subject of quartz I must re^er to the famous 

 mathematical theory of Fresnel, who endeavoured to explain 

 its action upon light by supposing that the plane-polarized wave 

 on entering it is split into two waves, consisting of oppositely 

 circularly-polarized light, which traverse the crystal with dif- 

 ferent speeds. On emerging they recimbine to form plane- 

 polarized light, the plane of which, however, depends on the 



Fig, II. — Model illustrating recomposiiion of rectilinear motion from two 

 opposite Circular mot'ons. 



retardation of phase between the two components. I here 

 introduce a mechanical model to illustrate one of the points in 

 this theory — namely, the recombination of two circular motions 

 to form a straight-line motion. These two disks (Fig. 11), which 

 turn in opposite senses, but at equal rates, represent two circu- 

 larly-polarized beams of light. The linkages, which connect two 

 pins on these disks, compound their moti )ns at the central point, 

 p, which executes, as you see, a straight line. But now, 

 suppose one of these circular m jtions to be retarded behind 

 the other, an effect which I can imitate by shifting one of 

 the pins to another position on the disk. Still the resultant 

 motion is a straight line, but it is now executed in a direction 

 oblique to the former. In other words, its plane has been 

 rotated. Of course this model must not be taken as establish- 

 ing the truth of Fresnel's ingenious theory : it is at best a rough 

 kinematical representation of it. 



Fig. 12. — Quartz crystal, showing: 

 characteristic facets : right-handed. 



Fig. 13. — Quartz cryst.tl, showing 

 characteristic facets : lett-handed. 



We have, however, the puzzling fact still to account for that 

 there should be two kinds of quartz crystals, right- and left- 

 handed. Sir John Ilerschel first showed that natural crystals 

 of quartz themselves often indicated their optit al nature, by the 

 presence of certain little secondary faces or facets which lay 

 obliquely across the corners of the primary faces. These are 

 indicated in the diagrams (Figs. 12 and 13), and may be seen in 

 two of the specimens of quartz crystals which lie upon the 

 table. The largest of these is right-handed. The wider 

 generalizations of Pasteur, respecting the cry-talline form of 

 optically active substances, show that those substances which 

 exercise an optical torque, whether as crystals or in solution, 

 belong to the class of forms which the crystallographer dis- 

 tinguishes as jossessing non-superposable hemihedry. In other 

 words, they all show s/icw symniet)-)', as if in the growth of them 



they had been built up in some screw-fashion around an axis, 

 and must therefore be either right-handed or left-handed screws. 

 By piling up a number of wooden slabs in skew-symmetric 

 fashion, I am able roughly to illustrate (Figs. 14 and 15) the 

 difference between the right-handed and the left-handed struc- 

 ture. It is a curious fact, if I am rightly informed, that down 

 to the present date the only substances possessing this skew 

 symmetry are natural substances ; that those which the chemist 

 can produce by artificial synthesis are all optically inactive. It 

 is perhaps equally significant that as yet no inorganic substances 

 have been found which will in the liquid state rotate the light. 

 This appea s to be a property possessed solely by certain com- 

 pounds of carbon. Quartz fused in the blowpipe or dissolved 

 in potash shows no trace of rotatory power. 



Yet we can have little doubt that this property is bound up in 

 the yet unravelled facts of atomic and molecular structure. In 

 the case of the liquids, such as turpentine and sugar solution, 

 there must be some skew symmetry in the grouping of atoms in 

 the molecule to produce the result. In the case of quartz, 

 there must be a skew in the building of the molecules — 

 there must, to borrow a phrase from the architect, be 



Fig. 14. — Skew-iymmetrical arrangement : right-handed. 



Fig. 15. — Skev. -symmetrical arrangement : left-handed. 



an oblique bonding of the minute bricks of which its trans- 

 parent mass is builded. Though we cannot even rebuild 

 it from its solution, we know this must be so, for we can 

 reproduce all the optical phenomena which it exhibits by an 

 actual skew building of thin slices of another non-mtatory 

 crystal. Here is an artificial object (I built it myself) con- 

 slructed on Reusch's plan, from sixteen thin slips of mica built 

 up in staircase fashion — right-handedly — one ab:.ve the other, 

 and set symmetrically at equal angles of 45° to one another, the 

 whole set making a cork-screw of two complete turns. In the 

 lantern it behaves just as a quartz of about 9 millimetres thickness 

 would do. It even gives tolerably perfect rings, as quartz does, 

 when viewed by convergent light. 



I must now pass hastily onwards to the great discovery of 

 Faraday. Here (Fig. 16) is a magnetizing coil of wire, M, 

 having about 8300 turns, and enclosed in an iron jacket. When 

 it is traversed by a powerful electric current from the dynamo 

 machine, it produces an intense magnetic field along its axis. In 

 this axial position lies a bar of heavy glass, not quite so dense 

 as that which Faraday himself used, but nearly so. The bar 

 lies along the line of light from our lantern, but the polarizer, P 



