lo Lankshear, Quantitative Absorption Spectra. 



K=ec in the expression of Beer's 1^ = 1 . lO'^""^, where e is 

 the extinction coefficient and c the concentration in gram 

 molecules per litre. It is thus possible hy a choice of 

 suitable thicknesses and exposures to evaluate e over the 

 whole range of the absorption band. 



M. Henri and his co-workers have examined a num- 

 ber of substances and have drawn several generalisations 

 of interest which will be considered in a later communi- 

 cation. Their method is open to criticism, and it is 

 computed that errors of the order of 30 per cent, may 

 affect their figures. 





^':, 



Fig 1 



Fig I. 



The first and most serious of these errors relates to 

 the standard spectrum. They made the exposure as 

 exact as possible by means of the rotating sector com- 

 monly used in photographic measurements, but there is 

 always the possibility of variation in the light source, 

 which is the condensed cadmium and iron spark. Further, 

 the accuracy of Schwartzschild's formula is assumed over 

 the whole range of the spectrum, for any plate, and for 

 any value of /. These defects are overcome in the 

 method about to be described. In this latter, due to 



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