8o8 



NATURE 



[December 16, 1922 



the danger of confusion arising from the existence 

 of contradictory conventions such as those of Biot 

 and Herschel, because, on page 86, he writes : — 



" There are two varieties or kinds of circularly 

 polarised light which have been respectivelv dis- 

 tinguished by the names of dextrogyrate or right- 

 handed, and laevogyrate or left-handed. 



" In one of these the vibrations are formed in an 

 opposite direction to those in the other. Unfortu- 

 nately, however, writers are not agreed on the applica- 

 tion of these terms ; and thus the polarisation, called, 

 . right-handed, is termed, bv Herschel, left- 

 handed, and vice versa. There is, however, no differ- 

 ence as to the facts, but merely as to their designation. 

 If, on turning the analysing prism or tourmaline 

 from left to rigid, the colours descend in Newton's 

 scale, that is, succeed each other in this order — red, 

 orange, yellow, green, blue, indigo, and violet, Biot 

 designates the polarisation as right-handed, or +, 

 or /*; whereas if they descend in the scale bv turning 

 the analyser from right to left, he terms it left-handed, 

 or - , or *v Sir John Herschel, on the other hand, 

 supposes the observer to look in the direction of the 

 ray's motion. Let the reader, he observes, ' take a 

 common corkscrew, and holding it with the head 

 hi in, let him use it in the usual manner, as 

 if to penetrate a cork. The head will then turn 

 the same way with the plane of polarisation as a rav 

 m its progress from the spectator through a right- 

 handed crystal may be conceived to do. If the thread 

 oi ili'' corkscrew were reversed, or what is termed a 

 left-handed thread, then the motion of the head, as 

 the instrument advanced, would represent that of 

 the plane of polarisation in a left-handed specimen 

 of rock-crystal.' 



1 shall adopt Biot's nomenclature, and designate 

 the polarisation right-handed or left-handed according 

 as we have to turn the analysing prism to the right 

 or to the left to obtain the colours in the descending 

 order." 



We have in these paragraphs a verv clear and 

 unambiguous statement of the two conventions. 

 Biot's is finally adopted and used consistently 

 throughout the book. It will be noted, however, 

 that Pereira speaks of colours which succeed each 

 other in the order, red, orange, yellow, etc., as descend- 

 ing in Newton's scale. 



A second and greatly enlarged edition of Pereira's 

 book, edited by the Reverend Baden Powell, appeared 

 in 1854, after the author's death. In this edition 

 the above paragraphs remain substantially the same, 

 except that the words " the colours descend in 

 Newton's scale, that is, succeed each other in this 

 order, red, orange, yellow, green, blue, indigo, and 

 violet," in the first edition, are replaced bv these 

 won Is in the second edition (see p. 253) — " The 

 colours descend in the order of Newton's scale — that 

 is, succeed each other in the order of the colours of 

 their plates, reckoning from the central black as 

 the highest point." 



It will be noticed that the enumeration of the 

 colours, red, orange, yellow, etc., has been replaced 

 by the words " succeed each other in the order of 

 the colours of their plates, reckoning from the central 

 black as the highest point," so that we must seek 

 further for information as to the meaning of the words 

 " descend in the order of Newton's scale " as used in 

 the second edition. And turning to page 256, we 

 read : — 



Thus, suppose we turn the analyser right-handed, 



that is, as we screw up, the colours succeed each 



other, with a certain thickness of the crystal, in 



this order — red, green, fm fie, red 



rid so on, in the ascending order of Newton's 



on the colours of thin plates, before given." 



NO. 27/2, VOL. I io] 



So that in the second and first editions, the word 

 descend " has contradictory and opposite meanings. 

 In the first edition it refers to colours succeeding 

 one another in the order, red, yellow, green, and 

 blue, whereas in the second edition the word ' ' ascend- 

 ing " is used to denote the same order of colours. 

 The result is that while Biot's convention was given 

 clearly and correctly in the first edition, and used 

 consistently, that given as Biot's convention in the 

 second edition is, unfortunately, not Biot's but 

 Herschel's convention, but both are used, with the 

 result that the student gets hopelessly puzzled. 



In Dr. Tutton's book, in spite of the warning in 

 the footnote quoted, the first and also the second 

 editions have apparently been written consistently 

 with the Biot convention. In the first edition, for 

 example, at pp. 802-803 and in the second edition at 

 pp. 1082-1083, it is stated that " a slight rotation of 

 the analyser from the position for the violet transition 

 tint, to the right (clockwise) or left (anti-clockwise) 

 according as the crystal is right-handed or left- 

 handed, causes the colour to change to red (first 

 order). On the other hand, a rotation of the analyser 

 contrary to the rotary character of the plates causes 

 the violet transition tint to change to blue or green 

 (second order)." This statement, it will be seen 

 upon consideration, can only be true of the right- 

 and left-handed crystals shown by Figs. 344 and 345 



of the first edition (here reproduced in Fig. 2) and 

 Figs. 311-312 of the second edition, upon Biot's con- 

 vention. Upon Herschel's convention the words 

 " right (clockwise) " and "left (anti-clockwise) " should 

 be transposed in the above quotation. 



Any attempt to revive Herschel's convention 

 should, I think, be resisted. Simple experimental 

 facts should be capable of description in clear and 

 unambiguous language, and this, as has been shown, 

 is not likely to be achieved so long as two conventions, 

 in such a simple matter, are tolerated. The fact that 

 Herschel himself brought his convention into line with 

 crvstallographic nomenclature by calling what is now 

 universally accepted as a right-hand crystal, a left- 

 hand crystal, has been overlooked. The crystallo- 

 graphic conceptions of right- and left-handed crystals 

 are not likely to be changed now, so that the adoption 

 of the Herschel convention by any writer will, or 

 should, necessitate the definite statement that accord- 

 ing to this convention a right-hand crystal is made 

 up of left-hand quartz. Dana, it is true, in his 

 System of Mineralogy " gives Herschel's com ention, 

 but he accepts at the same time the usual definition 

 of right- and left-hand crystals, so that a right-hand 

 rystal, according to him, is left-hand optically. It 

 should be remembered, however, that the last edition 

 of Dana appeared thirty years ago. Later writers such 

 as Miers, Johannsen, Duparc, and Pearce, and many 



