!33 



VI. Mixture of ammonium tartarate and chromium alum solu- 

 tion {2%ese, p. 72); ;.< /o. 



Observed rotation: qX). ip<a) 



Observed revolution: left-handed 



VII. Mixture of sodium tartarate and chromium alum solution 

 {These, p. 74). 



a) Z^ ?.Q b) i <C ^0 



Observed rotation : ()<I0, ^/'>0 (>>'0, t/i<10 

 Observed revolution: left-handed * left-handed * 



With these facts before us, there can be no doubt that the rule 

 .as now corrected rests upon a foundation of truth. In the above 

 enumeration not a single instance can be adduced that would not 

 fit with the rule given in Art. 4. The evidence then, so far as it 

 goes, is in favour of the formula which affords a means of expres- 

 sing that hitherto wanting relationship. 



§ 6. On p. 65 of Mr Cotton's These (p. 407 of the Annales 

 reprintj measurements, for different wave-lengths, of the rotation 

 and of the ellipticity imparted to light by a 10^/g solution of double 

 chromium and potassium tartarate are recorded. In a previous 

 paper ^) this very instructive series of experiments was chosen as 

 a convenient example of the application of the general theory. In 

 the course of the calculations two particular figures in the table, 

 •one value of ip and one of g). were observed to be at variance 

 with the graphical representation of the results (p. 66 of the These', 

 Annales, p. 408); the discrepancy was attributed to misprints in 

 the table {I. c. p. 776, p. 780) and the diagram adhered to repeat- 

 edly. On receiving a copy of my paper, Mr Cotton found, 

 however, that in one of the two cases the diagram, and not the 

 table, was in error. On this point Mr Cotton has been good 

 ■enough to send me the following information which is here printed 

 with his kind permission: «J'ai consulte mes cahiers d'experience 

 «pour voir ce qu'il en est des fautes que vous signalez pour les 

 <mesures rapport^es pour la longueur d'onde 562. Vous avez par- 

 ^faitement raison pour I'une d'elles : il faut lire, dans la table 

 -sdonnant les valeurs de Tangle (p. 4° 46' au lieu de 4*^16'. Mais 



1) Bulletin Int. for October 1908, p. 775. 

 Btillctin III. 3 



