WILLIAM R. AMBERSON 523 



taken up in a double pipette controlled by a single bulb. Pressure 

 upon this bulb throws an enzyme solution into each tube at exactly 

 the same instant, and with considerable and identical force. In 

 some few experiments I have relied upon the agitation thus produced 

 to secure mixing, and the resulting records have given every evidence 

 of uniformity and consistency. In order, however, after the usual 

 chemical procedure, to insure thorough and continuous mixing of 

 the reactants, I have in most cases stirred both tubes by means of 

 two small motor driven propellers, run at identical speeds, and dipping 

 just below the surface of the reacting solutions. The blades are 

 arranged to throw the surface film continually into the depths. The 

 oxygen supply is thus also replenished, but, as will be seen below, this 

 factor does not appear to affect the reaction velocity. 



Eastman cine-negative film is 35 mm. wide. The lower record 

 6 mm, wide, runs parallel to the lower edge of the film, and at a 

 distance of some 4 mm. from it, the upper record, of the same width, 

 runs through the middle of the film, its lower edge at 4 mm. distance 

 from the lower record. The upper third of the film is thus left blank, 

 and upon it later there is impressed a series of caKbration exposures, 

 shortly to be considered. 



It can be easily seen that in a photographic study in which quanti- 

 tative measurements are desired, an accurate knowledge of the relation 

 between the intensity of the incident Hght and the resulting photo- 

 graphic density is essential. I have made no attempt to secure 

 information of this sort in any absolute terms, although such data 

 would undoubtedly be most interesting if it could be readily 

 obtained. Certain difficulties have made impossible the gaining 

 of such absolute determinations. The whole study has been one of 

 purely relative values. 



A host of studies upon the physics and chemistry of the photo- 

 graphic process have given rather extensive information as to the 

 intensity-density relationships. Those who are interested in a 

 detailed discussion of this subject may consult the book of Sheppard 

 and Mees.^ The curve of blackening of a film or plate assumes a 

 characteristic elongated S-shaped form when the logarithms of the 



^ Sheppard, S. E., and Mecs, C. E. K., Investigations on the thcor>' of the photo- 

 graphic process, London, New York, Bombay, and Calcutta, 1907. 



