432 M. G. Quincke on the Refractive Index of the Metals. 



have attempted to combine both cases in the figure, where the 

 globule a may be either a bubble of PH 3 about to burst and 

 project a ring of smoke into the air, or it may be a drop of liquid 

 about to descend in water, benzole, ether, &c, as in the previous 

 experiments. The ring of smoke, or the liquid ring, acts as if 

 rolling up or down the inside of a hollow cone, and the direction 

 of rotation of the particles will be found according to this view. 

 In both cases the tendency of the ring is constantly to enlarge 

 by diffusion, and the rate at which it does enlarge is regulated by 

 the resistance of the liquid column or of the air. The resistance 

 of the liquid column is much greater for the liquid ring than of 

 the air for the smoke ring ; and hence the liquid rings do not ex- 

 pand much, while the smoke rings expand greatly. But the resist- 

 ance, whatever its amount, must clearly be applied to the outer 

 surface of the ring ; or the ring may be said to bear on the sur- 

 rounding medium by its outer surface, which would be equiva- 

 lent to its rolling up the inner surface of a hollow cone. In 

 the figure, the straight vertical arrows show the motion of the 

 ring up or down, the oblique arrows give the direction of the 

 resultant of the forces acting on the ring, and the direction of 

 the resistance of the medium to this resultant, while the curved 

 arrows show the direction in which rotation must occur accord- 

 ing as the general direction of the movement of the rings is 

 upwards or downwards. 



The separate ring of arrows is nearly the same as that given in 

 a little book of mine published by the Christian Knowledge So- 

 ciety in 1848, and intended to illustrate the formation of a ring of 

 smoke as projected from a factory chimney (here representing a 

 liquid ring or an air ring, according to the position in which the 

 plate is held) . It explains the appearance in the liquid rings 

 of two Ionic-like volutes on either side of the ring, and pro- 

 duced by the perspective of a number of rings seen through, 

 or nearly through, each other, while at the front and back the 

 edges of single rings only are seen. 

 King's College, London, 

 April 1864. 



LXVI. On the Refractive Index of the Metals. By Gr. Quincke*. 



IN a former memoir " On the Optical Properties of the 

 Metals," I have called attention to the fact that the re- 

 fractive index of the metals, especially of silver and gold, can be 

 less than If. This followed from the application of Cauchy 

 and Eisenlohr's theory to the constants of the elliptical polariza- 

 * Translated by Professor Wanklyn from PoggendorfFs^wttaZeWjVol.cxix. 

 part 4 (1863), p. 599. 



t See Phil. Mag. vol. xxvii. p. 161. 



