340 Dr. Ketteler on the Dispersion of Light in Gases. 



and here expresses the ratio of the shortening of the wave-lengths 

 corresponding to two definite times of vibration, may henceforth 

 be regarded as a measure of the amount of dispersion. Accord- 

 ing to this, the dispersive force must be independent of the 

 density, and essentially connected with the properties of the 

 molecules. 



For the sake of perspicuity, I have collected in the following 

 Table the whole of the observations made upon the above-men- 

 tioned gases : — 



To 10,000 red bands correspond — 



Yellow 

 Bands. 



Green 

 Bands. 



is 



11389-53 12546-36 



11428*6 



12631-6 

 12659-6 

 12698-1 

 12705-7 



When one of the interfering rays 

 geometrically retarded by the length- 

 ening of its path in air .... 

 When one of the interfering rays is phy- 

 sically retarded by increase of density in 

 Air and carbonic acid gas . . . 



Hydrogen 11441-4 



Cyanogen 11460-5 



Sulphurous acid gas .... 11463-4 



If the numbers of the second column are calculated from those 

 of the third, as they easily can be (see below), the mean differ- 

 ence between calculation and observation amounts to about seven 

 units in the sixth place. 



The great dispersive power of hydrogen is especially interesting. 



It now only remains to determine the indices of refraction for 

 some one colour (for example, that of the sodium-light) in order to 

 know them for the two other colours also. For this purpose, expe- 

 riments were made in which the increase of pressure correspond- 

 ing to the displacement of every twenty-five bands was observed. 



The following results obtained in this manner with air are 

 given as examples : — 



m. 



Pi- 



Pv 



m 



p J. 

 m 



Temperatures. 





millims. 



millims. 



millims. 



millims. 









- 0-33 



- 0-98 









25 



+99 09 



+98-78 



3-9784 



3-9644 



At the beginning: — 



50 



199-42 



198-92 



3-9966 



39962 



Tubes 18°-35 



75 



29919 



298-71 



3-9952 



3-9941 



Manometer 18°-42 



100 



39901 



398-62 



3-9950 



3-9913 





125 



498-85 



498-30 



3-9950 



39925 



At the end : — 



150 



598-34 



598-27 



3-9927 



3-9933 



Tubes 18°-60 



175 



697-76 



697-58 



3-9906 



3-9901 



Manometer 18°-50 



200 



797-58 





3-9911 







The column headed m gives the number of bands, and the 

 columns headed p the corresponding excesses of pressure, the 



