836 Mr Herschel cm the Devlaticyns from Newtoi'Cs Sdale 
in their action on coloured light, precisely the order and pro- 
portions stated in the Table of Newton, for the colours of thin 
plates of air. This coincidence is certainly extremely remark- 
able, supposing it rigorously exact * ; and antecedent to further 
experience, would appear to authorise the conclusion, that the 
proportional lengths of the periods performed by differently co- 
loured rays within crystallized bodies, depend essentially on the 
nature of the rays themselves, and not at all on the interior con- 
stitution of the crystal. Indeed, in a case very analogous, M. 
Biot himself has attributed great and decisive weight to a pre- 
sumption resting on the very same grounds. I allude to his 
Memoir on the Rotatory Phenomena exhibited by rock-crystal, 
and certain liquids, where having observed that, in the former 
^ubstance, the rotatory velocities impressed on the planes of po- 
larization of differently coloured rays, are inversely as the squares 
of the lengths of their fits ; he argues, that this relation being 
independent of any datum involving the peculiar constitution 
of rock-crystal, ought to be the general law for all other sub- 
stances which possess the rotatory property* “ Ou pourrait con- 
siderer d’abord que la rotation dans le cristal de roche s’etant 
trouvee reciproque aux carres des longueurs des acces des divers 
rayons simples, cette loi se presente cornme une propriete des 
rayons memes, et non comme une resultat dependant de la na- 
ture des corps qui agissent sur eux. Ou doit done s’attendre, 
d’apres cette remarque, que la meme loi subsistera dans toutes 
les substances, comme on y voit se maintenir les rapports des ac- 
ces memes dont la seul longueur absolue varie.” 
However convincing this line of argument may appear, and 
however exactly supported by experiment in the case of the ro- 
tatory phenomena, its conclusions are not verified in that of the 
polarized rings, to which it nevertheless applies with as much 
or greater force, as in the other instance ; and this may serve to 
shew how very cautious we ought to be in our attempts to ge- 
neralise antecedently to experience in this branch of optical sci- 
ence. In the paper above alluded to, I have demonstrated that 
• M. Biot’s conclusion was deduced principally from sulphate of lime ; but 
even in this substance the tints are not those of Newton’s Scale. Round the result- 
ant axes the tints are of a very peculiar nature, as I shall have occasion to shew 
in a separate Memoir on this subject. — D. B. 
2 
