December 16, 1922] 



NA TURE 



807 



Spinulosa, as the more primitive. This opinion is also 

 adopted by Hamann in " Bronn " and by Gregory 

 in Ray Lankester's " Treatise on Zoology." Prof. 

 MacBride alone, and, among late specialists in Aster- 

 oids, Perrier, have expressed the view that the 

 Spinulosa are the more primitive of Asteroids. Is 

 Prof. MacBride then not perhaps attaching somewhat 

 too much value to his own opinion when he states 

 that " all admit " the Spinulosa to be the most 

 primitive group of Asteroids — with myself alone as 

 an absurd exception ? 



The question which group of Asteroids is the most 

 primitive may not yet be definitely solved. If. 

 however, — as nearly all admit — the Astropectinid 

 forms are the most primitive, the conclusion is in- 

 evitably that the Brachiolaria, occurring — so far as 

 evidence goes — only in the more specialised groups, 

 the Spinulosa and Forcipulata, is a specialised larval 

 form and its sucking disk a specialised, later acquired 

 structure. Then this sucking disk is not homologous 

 with the crinoid stalk, and its use in phylogenetic 

 speculations is unjustified. 



To Prof. MacBride's suggestion that my views 

 would have some more value if I " had worked out 

 with thoroughness the complete life-history of any 

 Echinoderm," and to his protest against " the idea 

 that those interested in Echinoderms agree with the 

 over-estimate of the importance of trifling peculiar- 

 ities in the structure of pedicellariae in which Dr. 

 Mortensen indulges " Dr. Bather has already kindly 

 replied. In order not to make this belated reply too 

 lengthv I shall then not take up these challenges at 

 present. Th. Mortensen. 



Zoological Museum, Copenhagen. 

 November 22. 



Rotary Polarisation of Light. 



In the second edition of Dr. Tutton's monumental 

 work on " Crystallography and Practical Crystal 

 Measurement," a question of some interest to 

 crystallographers and physicists is raised in an acute 

 form by a footnote at the bottom of page 1082, 

 which reads : — 



" Considerable confusion has been introduced into 

 the subject of optical rotation by the fact that 

 chemists, in their use of the polarimeter for the 

 determination of the rotation of the plane of polarisa- 

 tion by opticallv active substances (chiefly liquids 

 or solids in solution, but occasionally the solids 

 themselves), have adopted a different convention, 

 as regards the sign of the rotation, to that employed 

 by physicists and crystallographers, who refer to the 

 actual occurrence in the crystal itself. For instance, 

 the right-handed quartz of the crystallographer 

 actually rotates the plane of polarisation of light 

 in the opposite direction to the so-called dextro- 

 camphor of the chemist. The latter regards a 

 rotation as right-handed or dextro when it appears 

 clockwise to the observer looking through the eye- 

 piece of the polarimeter. But the crystallographer 

 regards himself as travelling with the beam of light, 

 that is, as looking along the direction of propagation 

 of the light : if the movement of the light in the 

 crystal is like that of a right-handed screw, clockwise, 

 the crystal is right-handed or dextro-gyratory, and 

 if the light moves in left-handed screw fashion, anti- 

 clockwise, the crystal is laevo-rotatory or left-handed. 

 It is very important that this should be quite clear." 



This question as to the precise meaning to be attached 

 to the words " right-handed rotation " has been 

 responsible for a certain amount of misunderstanding 

 and confusion in text-books on mineralogy and 

 physics for nearly a hundred years, and from Dr. 



NO. 2772, VOL. I IO] 



Tutton's footnote it would appear that it is still 

 unsettled. 



Now the facts are simple. In 181 3, the famous 

 physicist Biot read a paper before the Institute of 

 France ' in which he described a number of experi- 

 ments that he had made upon plates of rock-crystal 

 cut perpendicularly to the axis of crystallisation. 

 In carrying out this work Biot made the important 

 discovery that there are two kinds of quartz — one 

 in which the plane of polarisation is rotated to the 

 right, while in the other the rotation is to the left. 

 In carrying out these experiments Biot used a table 

 polariscope, and adopting as a standard succession 

 of colours that in which they ascend in Newton's 

 scale, namely, red through yellow and green to blue, 

 he found that a rotation of the analyser from left 

 to right, that is in a clockwise direction, gave the 

 standard succession for one kind of quartz, while 

 an opposite rotation gave it for the other. The 

 first rotation he spoke of as right-handed and the 

 second, consequently, as left-handed. The experi- 

 ments were subsequently carried out upon a consider- 

 able number of liquids and the convention of direction 

 of rotation referred to was applied consistently. 



In 1820, that is, seven years after Biot's discovery 

 of right- and left-handed rotation of the plane of 

 polarisation, Herschel read a paper before the Cam- 

 bridge Philosophical Society 2 in which he announced 

 his discovery that the direction of rotation of the 

 plane of polarisation in quartz is indicated by the 

 disposition of certain crystal faces. 



Unfortunately, however, Herschel was not satisfied 

 with Biot's convention, and he proposed to substitute 

 for it one in which the observer was supposed to be 

 looking along the beam of light in the direction in 

 which the light was passing. While Biot, as it 

 were, looked at an internally illuminated clock-face 

 from the outside, Herschel preferred to look at it 

 from the inside. Biot's right- 

 handed thus became Herschel's 

 left-handed rotation. Herschel, 

 however, was consistent. He 

 called the crystal which gave a 

 right-handed rotation according 

 to his convention, a right-hand 

 crystal, and in giving the results 

 of Biot's experiments on liquids, 

 he changed the signs in order to 

 bring them into accordance with 

 his own convention. Thus ac- 

 cording to Herschel cane sugar 

 in solution rotates the plane of 

 polarisation to the left. 



Fig. 1 is a reproduction of 

 the figure given in Herschel's 

 original paper and reproduced 

 in the article " Light " in the 

 Encyclopaedia Metropolitana 

 (1830). In this article it is 

 stated that the figure " repre- 

 sents a right -hand crystal." 

 It is important to note here Fk _ I _ Aright . 1|i 

 that in practically all modern (Herschel). 



books this figure illustrates a 



left-hand [twin] crystal. The confusion resulting 

 from Herschel's attempt to substitute his convention 

 for that of Biot was soon apparent. 



In 1843 a book entitled " Lectures on Polarised 

 Light delivered before the Pharmaceutical Society 

 of Great Britain " appeared. This admirable little 

 book of some hundred pages was written by 1 >r. 

 Pereira. Now Dr. Pereira was evidently alive to 



physiques de 



' ■■ Memoires de la das 

 I'lnstitut imperial lie Fran 

 - Trari 



nathematiques e 



I ':. I. pp. 263-4. 

 vol. i. p. 4 I 



