S 1 2 Cambridge Philosophical Society. 



Btechnum boreale. Mr. Don only notices this plant because he 

 finds that it is still referred by most botanists to Blcchnum, although 

 it properly belongs to Lomaria, as Mr. Brown long ago suggested, 

 agreeing remarkably with that genus, in habit, in having two kinds 

 of fronds, in its marginal indusium, &c. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



A meeting was held on Monday evening the 17th of February, 

 Dr. Clark, V.P., being in the Chair. Among the presents was a Pro- 

 teus anguinus, offered by Mr. Lunn, with some observations on the 

 history of our knowledge of the animal. Professor Miller commu- 

 nicated a notice of some optical experiments, by which it appeared 

 that the lines seen in the vapour of bromine and iodine are identical 

 in position ; and that the vapour of perchloride of chrome exhibits 

 lines apparently equidistant, much closer and fainter than the bro- 

 mine lines, but occupying the same part of the spectrum. Mr. 

 Whewell read a memoir on the nature of the truth of the laws of 

 motion, tending to show that these laws may be demonstrated in- 

 dependently of experiment so far as their terms go ; but that the 

 meaning of the terms must be assigned by a reference to experi- 

 ment. 



A meeting was held on Monday evening, March the 3rd, Dr. 

 Clark, V.P., being in the Chair. A memoir was first read by the 

 Rev. J. Challis, containing new researches in the theory of the motion 

 of fluids. 



The Rev.T. Chevallier described experiments which he had made 

 on the polarization of light by the sky. The general results were, 

 that light is polarized by the clear sky ; that the effect begins to be 

 sensible at points thirty degrees distant from the sun ; and that the 

 greatest quantity of polarized light proceeds from points at ninety 

 degrees distance from the sun, a fact which seems to indicate that 

 the reflection, which occasions the polarization, takes place at the 

 surface of two media as nearly as possible of the same density. It 

 was also stated that though the light of the moon or of clouds shows 

 no trace of polarization, a fog, when on the point of clearing off, lets 

 polarized light through when its breaking up has not yet begun. 

 Mr. Chevallier remarked that he had not detected any appearances 

 of polarization by transmission, though, as was mentioned by an- 

 other member, M. Arago has stated that he had observed, within 

 a certain small distance of the sun, that the light was polarized in the 

 opposite plane to that at a greater distance. 



A meeting was held on Monday evening, March the 17th, Profes- 

 sor Airy, one of the Vice-Presidents, being in the Chair. Mr. Power 

 gave an account of his views concerning the cause of the pheno- 

 mena of exosmose and endosmose ; which it appeared by his calcu. 

 lations may be accounted for by the effect of forces similar to those 

 which produce capillary phaenomena. 



Professor Henslow gave an account of the speculations of M. 

 Braun respecting the spiral arrangements of the scales on the cones 

 of pines, illustrated by drawings and additional observations. 



