208 



Chapter XIII 



scale, and is the measure of the degree of shifting of the band, which 

 in its turn is a measure of the percentage of CO haemoglobin present. 



I do not propose to enter here into a description of the manipula- 

 tive details, such as screening fluids for blocking out parts of the 

 spectrum, not germane to the inquiry. For these I must refer the 

 reader to the original papers of the author, nor do I propose to 

 discuss the abstract foundation of the method from the point of 

 theoretical optics. 



I will confine myself to some criticisms of its practical sufficiency 

 as a piece of laboratory technique. 







FIG. 101. The apparatus as employed for the calibration of Hartridge's spectroscope 



in plan and in elevation. 



Like the carmine method it is a subjective method, it depends 

 upon the matching of two things as seen by the eye. Other things 

 being equal it is an easier proposition to put two dark lines into 

 exact juxtaposition than to match two tints to within 2/ . Much 

 depends upon the total range of difference in each case and the errors 

 involved in the two operations are not easy to compare. 



Hartridge has been at great pains to define the subjective element. 

 He has spared no labour to satisfy himself that the method gives 



