232 Sir John Conroy. On the, Refractive Index of [June 20, 



corresponds to 4 units in the sixth place of the refractive index ; as 

 the sets of micrometer readings were all fairly concordant, and their 

 probable errors less than I", the values of the indices were calculated 

 to six places. 



The temperatures at which the observations were made not being 

 identical for the two positions of the prism, it was thought that a 

 graphical method, though, from its nature, somewhat " arbitrary," 

 would give a more truthful result than any arithmetical process of 

 taking the means. The results were therefore plotted on paper 

 divided into squares of 1 mm., 0'2 of temperature being represented 

 by 5 mm. on the axis of abscissae, O00001 of refractive index by 4 mm. 

 on that of the ordinates ; and a curve drawn in the ordinary way. 



Owing to the scale on which the results were plotted, a good deal 

 of " judgment" was necessary in drawing the curve. It was there- 

 fore thought desirable also to plot the results on a smaller scale, so 

 far as the ordinates were concerned ; this was done, one unit in the 

 fifth place being represented by 2 mm., and another curve drawn. 

 The values of the index for different temperatures, as given by the 

 two curves were compared and were found to agree satisfactorily ; 

 and it therefore seemed probable that the curves were really fair 

 representations of the observations. 



In the first column of Table II the values of the refractive indices, 

 relative to air, for each degree as deduced from the curves, are given 

 to five places; in the second the values as found by Walter, and in 

 the third and fourth those for sodium light, given by Gladstone and 

 Dale, and Ruhlmann. 



Table II. Refractive Indices of Water. 



The values show that the refractive index of water, as was first 

 announced by Jamin, increases continuously up to the freezing point, 

 the rate of increase, however, seems to change about 4, the 

 temperature of maximum density, as was pointed out by Gladstone 

 and Dale. 



