and the Rotatory Dispersion Formula. 



421 



exclusive of the region between the D lines, with a fair degree 

 of accuracy. If, however, the constants a and b are calculated 

 from different pairs of readings on the same curve, different 

 values are found. On this account, it seems desirable to 

 redetermine the curves with still greater precision, which will 

 be done in the autumn. 



Vapour of a still greater density was investigated by a 

 photographic method. 



The spectrum of the light transmitted through the two 

 nicols and the sodium tube w T as photographed with the 

 three-prism spectrograph. Seven exposures were made on a 

 trichromatic plate, with the second nicol in seven different 

 positions. A sodium name, burning continuously in front of 

 the slit of the spectrograph, recorded the D lines on the plate. 

 The tube was heated electrically, and the density of the vapour 

 remained constant throughout the total time of exposure. 

 Each exposure amounted to less than half a minute. One of 

 these multiple photographs is reproduced on Plate V. fig. 2. 

 The picture may be said to show the dispersion curve. The 

 positions of the dark bands, which appear on each side of the 

 D lines, were measured on the dividing engine, and the wave- 

 lengths determined from the calibration curve of the spectro- 

 graph. For these dense vapours it was found that the 

 rotatory dispersion curves were symmetrical with respect to 

 a point midway between D 1 and D 2 . The dispersion should 

 therefore be well represented by a single-term formula 



in which \d = 5893. This was found to be the case, as the 

 following table shows. 



Value of constant a =135600. 



X. 



S Observed. 



o Calculated. 





o 



c 



5980 



5 



447 



5950 



10 



10-4 



5933 



20 



202 



5923 



40 



38-0 



5917 



00 



58-9 



5912-5 



90 



89-2 



5874 



90 



931 



5889 



43 



43-0 



5864 



40 



406 



5852 



20 



20-2 



5833 



10 



9-2 



5814 



5 



5-2 



The value of the constant was determined from the value 

 of 8 at wave-length 5869, as given by the curve. 



