ON THE REFRACTIVE INDICES OF CASEOUS POTASSIUM, ETC. 



145 



within ali. MII 2 per cent., an accurate record of the dispersion was hardly to be 

 expected. We have not overlooked the importance of determining the dispersion in 

 every case through a wide range of wave-lengths ; but the difficulty of obtaining 

 values sufficiently accurate to yield useful information when introduced into a 

 dispersion formula was found to be insurmountable with the present apparatus in the 

 time at our disposal. We hope, however, in the future, to make a further attempt to 

 obtain the dispersion with improved apparatus. 



The results appear interesting. In every case, except that of potassium, the 

 dispersion observed is normal in direction.* But its magnitude is very great. Thus, 

 in the case of tellurium, the refractivity varies from about 2370 for X = 6562 to 2620 

 for X = 5460, or nearly 10 per cent. For zinc the variation is about 7 per cent, for 

 the same range. 



Refraction and Refraction Equivalents. 



It is probable that the refractive indices for infinitely long waves would be 

 considerably smaller than those here observed. But even after making the most 

 liberal deduction, it is certain that the indices of zinc, cadmium, and tellurium are 

 higher than the highest (viz., that of iodine) of which we had previous knowledge. 

 It is interesting to compare them with the refraction equivalents of GLADSTONK and 

 others. This is done in Table VII. 



TABLE VII. 



h The apparent exceptions to this statement are attributable to errors of observation, 

 t 'Journal Chem. Soc.,' 1870, p. 101 ; 'Phil. Trans.,' 1870, p. 9. 

 J 'American Journal of Science,' 3, 29, 1885, p. 57. 

 'Pogg. Ann.,' 131, 1867, p. 125. 

 || 'Journal fur Praktische Chemie,' 31, 1885, p. 339. 

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