in the Velocity of Propagation of Light in Water. 267 



namely the refraction indices for seventy-six substances for the 

 Fraunhofer's lines A, D, H, mostly for three temperatures be- 

 tween 5° and 36° C, and, further, the indices for sixty-eight 

 liquids for intermediate temperatures, and for all the principal 

 lines of the spectrum. This investigation has, moreover, a greater 

 value than the preceding one, because it furnishes us with a 

 ready means of judging of the correctness of the experiments. 

 The determinations were made in a hollow glass prism, and the 

 refractive angle was assumed to be constant at about 61° at the 

 different temperatures; whereas in their previous investiga- 

 tion Gladstone and Dale sought to determine the angle of least 

 deflection at temperatures previously fixed upon, as 0°, 5°, 10°, 

 20°, &c. : these measurements, in their new investigation, were 

 made at the temperatures which actually presented themselves 

 during the process of heating or cooling. The latter method 

 must certainly give results of far greater accuracy than the former 

 one. They estimate the error in their thermal data at 1° or 

 2° C, which, however, in water of high temperatures causes a 

 variation of +8 units in the fourth decimal place. They give 

 the exactness of their angular measurements at +V, which cor- 

 responds to an error of +2 units in the fourth decimal place. 

 We thus see that a variation of + 8 units of the fourth decimal 

 place may be very easily accounted for. They give the name 

 " sensitiveness " to the alteration in the refractive index n with 

 the temperature within 10° C. ; and ft— -1 is called the refractive 

 energy. The diversity in the numbers given for the refractive 

 index for water for the D line for 0° is not explained. In 

 order to compare the agreement or disagreement of Gladstone 

 and Dale's observations for water with mine, their results in re- 

 gard to the D line have been marked with little crosses in Plate V., 

 in which the change of the index in regard to the temperature 

 is represented graphically. I shall return again in the sequel 

 to the view which these physicists take of the relation between 

 the density and the indices of refraction. 



Finally, I have to mention a series of experiments by Miit- 

 trich* on the alterations caused by heat in the indices of refrac- 

 tion in rape-oil and water. To determine these values, Muttrich 

 employed the alteration in the optical axis of Arragonite in the 

 liquid after its alteration in the open air had been very exactly 

 measured. The determinations gave results far too high, although 

 the observations for the sodium-line agree very well with the 



* " Bestimmung des Krystallsy steins und der optischen Coostanten des 

 weinsauren Kalinatrons, Einfluss der Temperatur auf die optischen Con- 

 stanten desselben und Bestimmung des Brechungsquotienten des Riiboles 

 und des distillirten Wassers bei verschiedenen Temperaturen, von A. Mut- 

 trich," Pogg. Ann. vol. cxxi. pp. 193-238, and pp. 298-430. 



