34 



ascribe an origin different from that assigned by Dr. Young. In 

 order to obtain a more distinct view of these colours. Sir David 

 Brewster employed, instead of the substances used by Dr. Young, 

 the white of an egg, beat up into froth, and pressed into a thin film 

 between plates of glass. From observations of the colours exhibited 

 by plates so prepared, and also by the edge of a thin film of nacrite 

 in contact with copaivi balsam, the author deduces the conclusion, 

 that all these phenomena, as well as those often seen in certain 

 specimens of mica through which titanium is disseminated, and also 

 in sulphate of lime, are cases of diffraction, where the light is ob- 

 structed by the edges of very thin transparent plates placed in a 

 medium of different refractive power. If the plate were opake, 

 the fringes produced would be of the same kind as those often 

 noticed, and which are explained on the principle of interference ; 

 but, owing to the transparency of the plate, fringes are produced 

 within its shadow ; and, owing to the thinness of- the plate, the 

 light transmitted through it is retarded, and, interfering with the 

 partial waves which pass through the plate, and with those which 

 pass beyond the diffracting edge with undiminished velocity, modify 

 the usual system of fringes in the manner described by the author 

 in the present paper. 



" Of such Ellipsoids, consisting of homogeneous Matter, as are 

 capable of having the Resultant of the Attraction of the Mass upon 

 a Particle in the Surface, and a Centrifugal Force caused by re- 

 volving about one of the Axes, made perpendicular to the Surface." 

 By James Ivory, K.H., M.A.. F.R.S. L. and Ed., Inst. Reg. Sc., 

 Paris, Corresp. et Reg. Sc. Gotting. Corresp. 



Lagrange, who has considered the problem of the attractions of 

 homogeneous ellipsoids in all its generality, and has given the true 

 equations from which its solution must be derived, inferred from 

 them that a homogeneous planet cannot be in equilibrium unless it 

 has a figure of revolution. But M. Jacobi has proved that an equi- 

 librium is possible in some ellipsoids of which the three axes are un- 

 equal and have a certain relation to one another. His transcend- 

 ental equations, however, although adapted to numerical computa- 

 tion on particular suppositions, still leave the most interesting points 

 of the problem unexplored. 



The author of the present paper points out the following property 

 as being characteristic of all spheroids with which an equilibrium is 

 possible on the supposition of a centrifugal force. From any point 

 in the surface of the ellipsoid draw a perpendicular to the least axis, 

 and likewise a line at right angles to the surface : if the plane pass- 

 ing through these two lines contain the resultant of the attractions 

 of all the particles of the spheroid upon the point in the surface, 

 the equilibrium will be possible, otherwise it will not. For the re- 

 sultant of the centrifugal force and the attraction of the mass must 

 be a force perpendicular to the surface of the ellipsoid, which re- 

 quires that the directions of the three forces shall be contained in 



