1912.] 



Measurement of Absorption Bands. 



137 



I &.C constant 



TslC constant 



T«J constant 



T' 

 J* 



C' 



c 2 



Fig. 4. — To show the Effect, of Changes— A, in the visual threshold ; B, in the initial 

 intensity of the light ; C, in concentration — on the Width of Absorption Bands. 



Fig. 5. — To show the Existence of an Optimum Concentration at which Best Measurements 



The alterations have been considered above as taking place uniformly 

 throughout the spectrum. It remains to consider the case when alteration 

 on one side only of a band is involved. 



It has been shown above that a rise in the threshold or a fall in the 

 intensity of the incident light both have the same effect, namely, to increase 

 the width of the bands. "Where one side only of a band is affected, it is 

 clear that a change in the mean wave-length of the band will be involved, 

 since one side of the band increases in width, the other remaining stationary. 

 This will cause the band as a whole to appear shifted towards that side on 

 which the intensity reaches its lowest, or the threshold its highest value. 

 This is shown diagrammatically in fig. 6. 



It will be remembered, when describing the personal and individual 

 variations in the wave-length of the bands of haemoglobin, that even under 

 the same conditions differences are still found to occur; these are to be 

 explained as being due to alteration in the threshold. For observers are 

 agreed that the threshold value for light of different wave-lengths is a factor 



+6 



are to be obtained. 



