" Law " in Physical Optics. 

 The square bracket is equal to 



339 





(*-/9), 



and it is under this head that the figures appear in the 

 following tables : — 



Carbon Disulphide. 



Temperature. 



/i 2 -l 



/* 2 +2 



^- 1 .(,-/3). 



»-j3 



v—0 



calculated. 



o 

 -20 



0-2788 



0-2821 



0-4277 



0-4259 



-10 



0-2793 



0-2835 



0-4349 



04341 







0-2798 



0-2847 



0-4424 







10 



0-2803 



0-2867 



0-4494 



0-4502 



20 



0-2808 



0-2875 



0-4578 



0-4581 



30 



0-2813 



0-2890 



0-4659 



0-4659 



40 



0-2819 



0-2904 



0-4746 



0-4735 



The experimental data are Ketteler's, and have already been 

 given. 



On comparing columns 2 and 3, it is seen that the figures 

 are remarkably close to one another. The difference is just 

 over 1 per cent, at —20°, and though it increases as the tem- 

 perature rises, it only just exceeds 3 per cent, at +40° C. 



Column 4 contains the values of the second term in the 



bracket, or -th of v — 6; and column 5 contains the "calcu- 



lated " values for the same expression. The calculation is 

 made by taking the value at 0° C. and multiplying by the 

 square root of the ratio of absolute temperatures. Thus the 

 value at — 20° is equal to 



0-4424 x 



?5* =0*4259. 

 274 



I was led to try this by the results obtained in the first part 

 of the present paper, and the extremely close approximation 

 of the calculated to the other numbers is surely of some 



/3 



will be referred to as the 



importance. The quantity 



interstitial space. " 



The next table contains data for substances already men- 

 tioned. 



2 A2 



