These numbers will be approximately converted into the corresponding wave-lengths in vacuo by multiplying them by 100029. 87. The formula established by the experiments of Biot and Arago for the index of refraction of air was t denoting the temperature Centigrade, a the coefficient of expansion 00366, and h the pressure in millims. of mercury at zero. As the pressure of 760 millims. of such mercury at Paris is 10136 × 10o dynes per square centim., the general formula applicable to all localities alike will be *0002943 1+00366 Р 10136 x 106 where P denotes the pressure in C.G. S. units. be reduced to the form This can velocity of light in air, and neglecting the difference of velocity between the more and less refrangible rays, we obtain the following quotients of velocity in air by wave 89. Dr. Hopkinson ('Proc. R. S.,' June 14, 1877,) has determined the indices of refraction of the principal varieties of optical glass made by Messrs. Chance, for the fixed lines A, B, C, D, E, b, F, (G), G, h, H,. By D is to be understood the more refrangible of the pair of sodium lines; by b the most refrangible of the group of magnesium lines; by (G) the hydrogen line near G. In connection with the results of observation, he employs the empirical formula μ − 1 = a {1 + bx (1 + cx) }, where x is a numerical name for the definite ray of which μ is the refractive index. In assigning the value of x, four glasses-hard crown, soft crown, light flint, and dense flint-were selected on account of the good accord of their results; and the mean of their indices for any given ray being denoted by μ, the value assigned to x for this ray is μ MF where μr denotes the value of μ for the line F. The value of as a function of A, the wave-length in 10-4 centimetres, was found to be approximately The following were the results obtained for the different specimens of glass examined :— Hard Crown, Ist specimen, density 2.48575. a = = 0'523145, b= = 13077, c = — 2.33. Means of observed values of μ. A 1511755; B 1513624; C 1'514571; E 1520324; D 1'517116; b 1520962; F 1523145; (G) 1'527996; G 1.528348; h 1530904; H1 1'532789. Soft Crown, density 2.55035. LIGHT. Dense Flint, density 3.65865. 71 INDICES OF REFRACTION FOR LIQUIDS. 90. The following values of indices of refraction for liquids are condensed from Fraunhofer's determinations, as given by Sir John Herschel ('Enc. Met. Art.,' Light, P. 415): A 1'6142; E 16465; A 14879; 91. The following determinations of the refractive indices of liquids are from Gladstone and Dale's results, as given in Watts' Dictionary of Chemistry,' iii., pp. 629-631 Sulphide of Carbon, at temperature 11o. B 16207; C 16240; Benzene, at temperature 10.5°. D 16333; H 1*7090. D 1'4975; 92. The following indices of doubly refracting crystals are from the table at the end of Lloyd's 'Light and |