WILLIAM R. AMBERSON 531 



at the same instant in the developing bath,!^ ^^^ rocked back and 

 forth, and up and down at random throughout the whole period of 

 development, to prevent local changes in developer concentrations. 

 I have not attempted to control temperature rigorously, although 

 the temperature of the bath is always about 18°C. Development 

 times are also varied to meet the varying intensities obtained in 

 different experiments. Development is in darkness, with the excep- 

 tion of occasional momentary viewings under the red light. 



The time relations of both calibration and moving records have 

 been so controlled as to make the two sets of exposures comparable. 

 It is known that the reciprocity law of Bunsen and Roscoe does not 

 hold for all values of time. It does hold, however, with close approx- 

 imation if the time is more than a very small fraction of a second, or 

 less than 100 seconds. Exposures along the moving records are for 

 about half a second, while calibration exposures have in no case 

 exceeded 100 seconds. In all records the great burst of light from 

 the calibration tube is over in a very few seconds. The two series 

 of densities may therefore be compared with a good degree of accuracy. 



I have entered into some detail in the above descriptions since they 

 appear rather necessary to a complete understanding of the problem, 

 and particularly since the theory and practice of photography in its 

 quantitative relationships is probably little known among biological 

 readers. For the benefit of these readers I will also discuss briefly 

 the methods employed in determining film densities. 



The Optical Pyrometer. 



The optical pyrometer is a photometer in which the intensity of 

 light emitted from the filament of a small pyrometer lamp is matched 

 against some other light source whose intensity it is desired to measure. 

 Fig. 3 indicates the details of the optical system. In my work I 



^* The following formula was used, diluted with an equal volume of water: 



Solution A. Water 64 oz. 



Hydroquinone 26 gm. 



Sodium sulfite 190 gm. 



Solution B. Water 42.4 oz. 



Potassium carbonate 152 gm. 



Potassium bromide 2 gm. 



