1912.] Measurement of A bsorption Bands. 139 



investigated without the use of spectral apparatus ; all the effects being 

 obtained by means of a- rotating disc calculated from the spectrophotometer 

 curve of any chosen band. Such a disc is shown in fig. 7. 



Fig. 7. — Diagram of Colour Disc used for imitating Absorption Bands and Effects of 

 Changes in Intensity and Threshold. 



Summary. 



(1) When absolute values of the CO saturation of haemoglobin are required 

 the spectroscopic method is complicated by the fact that each observer must 

 obtain a calibration curve for himself, and this should also be checked from 

 time to time against the blood from the particular source under examination. 



(2) These individual differences are due to the particular threshold values 

 at different wave-lengths. 



(3) The differences greatly detract from the value of accurate wave-length 

 measurements of absorption bands, both visual and photographic. 



(4) They do not, however, prevent accurate CO estimations being made, 

 provided that care be taken to work under standard conditions. 



(5) The effects are considered of variations, on one or both sides of an 

 absorption band, in — 



(a) The initial intensity of the light. 



(b) Value of threshold and adaptation of retina. 



(c) Contrast. 



VOL. LXXXVI. — E. M 



