RESPIRATION 421 



in any particular part of the spectrum; nor, finally, is any part of the 

 retina of one individual constant in its excitability for either white 

 light or colored light; the excitability of any one part being dependent 

 on side light falling on neighboring parts of the retina. The numerous 

 colorimeters, haemoglobinometers, etc., in which these sources of error 

 cannot be eliminated, are liable to very gross error, and appear to be 

 responsible for the discredit under which colorimetric methods suffer. 



With ordinary artificial light the differences in tint between the various 

 solutions become almost invisible. The dimmest daylight is better than 

 ordinary artificial light. With blue spectacles, however, the differences 

 become very evident, and fairly good results can be obtained in the 

 titration if the carmine is made of the proper strength (very much 

 stronger) to suit the light. Daylight is, however, far better. 



It is essential to accuracy with the carmine method that the carmine 

 solution should accurately match the standard blood solution in depth of 

 color. If the two do not correspond, it is easy enough to get a result: 

 for when the solution in one test tube is too deep in color it is only 

 necessary to incline the other in order to make its depth of tint appear 

 equal. The calculation of the percentage saturation becomes fallacious, 

 however, as is easily seen. One source of slight error in the titrations is 

 that a carmine solution which, when made up, exactly matches the blood 

 solution in depth, may, towards evening, be rather too strong, owing to 

 change in the light. This change can, however, be detected and rectified 

 very quickly, and attention would automatically be called to it by the 

 fact that considerably less carmine than before would suffice to produce 

 the tint of fully saturated blood solution. 



A further source of possible fallacy depends on the liability of blood 

 solution to decomposition. It is essential that the blood should be fresh, 

 and diluted with clean water in a perfectly clean vessel. Solution which 

 has been kept more than a few hours is useless. It may show no methae- 

 moglobin band, and appear to be unaltered; but on saturating it with 

 CO it will probably no longer give the full pink color of undecomposed 

 haemoglobin, and its depth of color will also be found to be less than 

 before. It is thus mixed with colored decomposition products which make 

 it useless for titration. The tint on saturation with CO affords a far more 

 delicate index than spectroscopic examination of the freedom of a blood 

 solution from pigments other than haemoglobin. 



When blood saturated, or partly saturated, with CO is diluted with 

 water, a small part of the CO must necessarily go into solution in the 

 water, as some dissociation of the CO-haemoglobin occurs. To demon- 

 strate this it is only necessary to saturate some blood with coal gas and 

 dilute some of it to 0.5 per cent with water. It will be seen at once that 



