IS Do.] Water at Temperature* between and 10. 229 



of the French physicist" (Jamin) "but show at the same time that 

 the remarkable reversion of the density at 4, is not without its 

 influence on the amount of sensitiveness ; the change of refractive 

 index between 10 and 5 being 0'0002, whilst that between 5 and 

 is only O'OOOL" 



They give a table for the values of the index to five places, for 

 A, D, and H at eight temperatures between and 11. 



In 1867 Riihlmann (' Pogg. Ann.,' vol. 132, p. 1 and 176) pub- 

 lished an account of observations he had made of the refractive index 

 of water at various temperatures ; he also used a hollow glass prism 

 and gives the values to five places, for lithium, sodium, and thallium 

 light from to 100. He states, " Der Brechungsindex des Wassers 

 nimmt stetig ab von bis 80 R., ohne bei dem Dichtigkeitsmaximum 

 irgend eine Abweichung von dem Aenderungsgesetze zu zeigen, mithin 

 die Fortpflanzungsgeschwindigkeit des Lichtes stetig zu." 



Lorenz (' Wied. Ann.,' vol. 1.1 [1880], p. 70) made observations by 

 an interference method, on the refractive index of water between the 

 temperatures of and 34 ; and Dufet (' Jour, de Physique ' (2), 

 vol. 4 [1885], p. 389) determined the index of water at temperatures 

 Jtbove 17 by the minimum deviation method and by an interference 

 method, and also calculated from the results obtained by other 

 observers the rate of change of the index with change of tempera- 

 ture. 



Ketteler (' Wied. Ann.,' vol. 33 [1888], pp. 353 and 506) repeated 

 Riihlmann's determinations for temperatures above 20, using a total 

 reflection refractometer, but did not make any observations at lower 

 temperatures. More recently still, B. Walter has published (' Wied. 

 Ann.,' vol. xlvi [1892], p. 422) a short account of some determinations 

 of the refractive index of water to five places between and 30 for 

 the D line, made, apparently, with great care by the minimum devia- 

 tion method. 



That the refractive index of water increases with the decrease of 

 temperature until the freezing point is reached, appears to be proved, 

 but as few determinations of the values of the refractive indices of 

 water near its point of maximum density have been published, I have 

 ventured, as the matter is one of considerable theoretical importance, 

 to bring before the Society an account of some determinations I have 

 recently made. 



The method employed was the ordinary one, the determination of 

 the angle of minimum deviation for a ray of definite wave-length 

 passing through a hollow glass prism containing water at a known 

 temperature. 



The goniometer used was made by Messrs. Troughton and Simnis, 

 it has an 8-inch circle divided into 10', and is read by means of 

 two micrometers, directly to 10", and by estimation to single seconds. 



