108 Prof*. Wood on the Dispersion, Absorption, 



absorption-spectrum of a thin crystalline film o£ the substance 

 between two quartz plates. A number of spectra were taken 

 with the nicol in different positions^, a comparison of which 

 gave unmistakable evidence that one band belonged to the 

 ordinary, the other to the extraordinary ray. In no case did 

 either of the bands disappear entirely, owing to the fact that 

 they overlap, which makes it impossible to have complete 

 transparency with the nicol in any position. No measure- 

 ments have as yet been made of the values o£ the extinction- 

 coefficient in different parts of the spectrum, owing to the 

 difficulty of preparing a film thin enough to transmit light 

 within the absorption-band. Though it is possible to get a 

 w^edge-shaped film between a lens and a flat plate which 

 transmits all wave-lengths to a certain degree in its thinnest 

 portions, a film of this nature is quite unsuitable for quanti- 

 tative measurements of the absorption. Probably by working 

 with solutions in glycerine of different concentrations and 

 thicknesses, a fairly correct idea of the absorption-curve 

 could be obtained by calculation, though this method would 

 be open to some objections. Until the substance has been 

 investigated in the infra-red, and until the dielectric constant 

 has been determined, the dispersion-formula cannot be applied 

 to it to the best advantage. At the present time, facilities 

 for investigating these two points are not at my disposal, but 

 I expect in the near future to investigate them. In the 

 meantime it is instructive to apply the formula to the results 

 which have already been obtained. 



The Dispersion Formula. 



For parts of the spectrum in which the extinction-coefficient 

 has a small value, the dispersion is represented by the formula 



2 -, . ^ mj,\^ 



1 + S 



\^-\Tc' 



in which X;t are the wave-lengths of the centres of the absorp- 

 tion-bands (nearly), and m-ic constants for these bands, the 

 summation being taken for all of the absorption -bands, 

 whether near or far removed from the portion of the spectrum 

 under consideration. If the bands are far remos^ed from this 

 region the fraction is practically equal to unity, and the 

 constants m' , irJ' , &c. have merely to be added or subtracted 

 according to whether they lie on the more refrangible or less 

 refrangible side of the spectral region under investigation, 



i. e. according as the sign of ■ ,^__ ,g is positive or negative. 



