416 PnOCEEDINGS OF THE AMERICAN ACADEMY 



in substance, that these are probably clue to the fact that, optically, 

 each molecule may be regarded as composed of groups of atoms, each 

 group possessing a specific optical character. So far as the interferen- 

 tial equivalents are concerned, further data are necessary to enable us 

 to test this explanation. 



Landolt has given the densities and indices of refraction of a number 

 of mixtures. 1 have not discussed these results fi-om my own point 

 of view, because since the publication of his work the progress of 

 organic chemistry has shown that many of the substances with which 

 Landolt dealt could not have been absolutely pure, though prepared 

 with great care for the special purpose of his investigation.* I think 

 I have shown that the so-called interferential constants possess a real 

 value as numerical characterisiics easily determined by measurements 

 of two indices of refraction and a single observation of density at the 

 same temperature. But the value of the new constants in quantitative 

 analyses can only be foirly estimated when we possess determinations 

 of indices and densities for a series of mixtures for which the propor- 

 tions, densities, and indices of the constituents are accurately known. 

 The time has also arrived when a much greater degree of accuracy in 

 the determination of indices of refraction is necessary. Even five 

 decimal places do not answer the present requirements of science. Six 

 are attainable with spectrometers reading to two seconds of arc. 



It is easy to see that the numerical value of an interferential con- 

 stant depends in part upon the angular distance of the spectral lines 

 between which the bands are counted. The lines Cand G are partic- 

 ularly well adapted for standard limits, as they are hydrogen lines 

 always obtainable by a small Ruhmkorfi^ coil and hydrogen tube. The 

 interferential constant may be taken as a measure of the dispersive 

 power of a body; and it is readily shown that with this measure, also, 

 the total dispersive power from A to // is the sum of the partial disper- 

 sions from A to B, B to C . . . G to H. The theory and construc- 

 tion of achromatic lenses might also be based upon this measure of 

 dispersive power, but it would probably possess no practical advan- 

 tages over the ordinaiy method. 



* I refer to tlie improvements in separating liquids of different boiling-points 

 introduced by Linnemann, — improvements wliich have shown tliat up to the 

 period of iiis work we liad no really accurate linowledge of the boiling-points 

 of a number of liquids long known to science, but never before obtained in a 

 state of perfect purity. 



