228 Dr. Alex. Hodgkinson on 



use his discretion as to the amount of absorbing medium 

 he employs, using such a quantity as will ensure the 

 splitting up of any compound band into any smaller bands 

 of which it may be constituted. But I need not say that 

 such method must necessarily ensure very variable results, 

 since, though each may have selected that thickness of 

 absorbing medium necessary to ensure the separation of a 

 compound band into its ultimate absorption bands, the 

 width of these bands, or, what is the same thing, the 

 position of these bands in the spectrum, is still a variable 

 factor, being dependent on the amount or thickness of the 

 absorbing medium. True, the results tend to become 

 uniform as the thickness of the medium becomes less, and? 

 if an infinitely thin film could be used, would be uniform ; 

 but the practical fact remains that all bodies become colour- 

 less long before such a point is reached, and, indeed, have 

 to be of considerable thickness before any absorption is 

 manifest by means of the spectroscope. And all practical 

 observers know the difficulty of measuring faint absorption 

 bands with ill-defined borders. It might be suggested that 

 the central point of a band could be taken as an indication 

 of its position in the spectrum, but against this is the fact 

 that a band usually fades off more gradually on one side 

 than on the other ; in other words, that the point of greatest 

 intensity of absorption is not in the centre of the band, so 

 that, in selecting the central point, we should be giving 

 preference to lines of less importance, so far as absorption 

 is concerned. Such a method, therefore, would not help 

 us. It is, moreover, impossible to secure any degree of 

 accuracy by fixing a standard strength, or thickness, of the 

 absorbing medium, although this is frequently suggested, 

 since, in the great majority of instances, we do not know the 

 percentage of colouring matter in the medium, and there- 

 fore equal thicknesses of such medium are not likely to 

 represent equal amounts. 



