380 Royal Socieli/. 



paper. By William Walker, Master R.N. ; communicated by the 

 President. 

 The author having shown in the former part of this paper, that the sta- 

 bility of compound vessels (see Phil. Mag. &Annals for April, p. 30 1 ,) as 

 far as depends upon their form, is a maximum when the displacement 

 thev produce by immersion in a fluid is equal to half their magnitude, 

 purposes in the present communication to prove that the same theory 

 is true of all rectangular vessels, whatever be their dimensions ; and 

 also the following proposition, namely, that when either the length or 

 breadth, or the length and depth are given, the maximum of stability 

 takes place when the ratio of the depth to the breadth is as one to 

 two. He enters at some length into the mathematical investigation 

 of both these propositions: he observes, however, that in the actual 

 construction of sliips, many considerations should be attended to, in- 

 dependently of the attainment of stability, — such as velocity of mo- 

 tion ; to obtain which condition, it is necessary to give the vessel as 

 much stability as possible, consistently with the least displacement, 

 so that the resistance to the direct passage of the vessel through the 

 fluid may be reduced to a minimum. When the quantity of materials 

 for building a ship of a given length is given, the maximum of ca- 

 pacity will be obtained when her breadth is double her depth. When 

 the breadth is given, by increasing the relative depth, the ship will, 

 when immersed to half her magnitude, carry less sail at small angles 

 of inclination, and vice versa. 



February 26th. — The reading of a paper was resumed and con- 

 cluded, intitled, " On the reflection and decomposition of light at the 

 refracting surfaces of media of the same and of different refractive 

 powers." By David Brewster, LL.D. F.R.S. Lond. and Ed. 



When white light is incident upon a surface which separates two 

 different media, the portion that is reflected should, according to the 

 Newtonian theory of light, preserve its whiteness, provided the thick- 

 ness of either of the media exceeds the 80,000,000th of an inch. But, 

 since the dispersive powers of bodies are different, it must follow, as 

 a necessary consequence, that reflected light can never, under any 

 circumstances, retain perfect whiteness, although the modification it 

 experiences is not of sufficient amount to become sensible in ordinary 

 experiments. The author, during his investigations of the laws of 

 polarization for light reflected at the separating surface of different 

 media, had occasion to inclose oil of cassia between two prisms of 

 flint glass ; and was surprised to find that the light reflected was of 

 a blue colour. The fact was new, but might be readily explained upon 

 the principle that, although the refractive density of oil of cassia 

 greatly exceeds that of flint glass for the mean rays, yet the action of 

 these two bodies is nearly the same on the less refrangible rays 5 

 hence it mav happen that a larger proportion of the former than of 

 the latter are transmitted, and the pencil formed by reflection will 

 then appear blue. The partial decomposition thus effected in the in- 

 cident rays will be the same in kind, though it may vary in degree, 

 at different angles of incidence, and cannot, therefore, give rise to 

 any variation of colour in the reflected rays, although they may dift'er 



in 



