﻿Theory of Photographic Exposure. 273 



dividing the observed values of log (1 — kjN) by the areas 

 the reader will find that the quotient increases considerably 

 and systematically, apart from a casual drop at the fourth 

 clump, throughout the whole series of the clumps. Thus, 

 the correction factor seems to come in quite spontaneously. 



On the other hand, there is nothing unlikely about the 

 values of either of the constants (12 a). Our units of area 

 being here square microns, we should have 57 millions light- 

 quanta per cm. 2 (about which judgment has to be suspended 

 until absolute energy measurements are available), and as 

 the cross section of the space occupied by each of them 

 (on an average) a little less than one-tenth of a square 

 micron or a diameter of about 0*35 micron. Since each is 

 presumably of about the order of a million wave-length 

 long, they are rather slender at that cross section, and, 

 instead of light parcels, as they were called above, would 

 perhaps deserve rather the name of light darts. In Einstein's 

 own theory there is nothing on which to base an estimate of 

 the volume occupied by a light-quantum, but on Marx's 

 less radical views this is about 8.10" 7 cm. 3 for D-light and 

 proportional to \ 4 , and therefore in our case (narrow blue 

 spectrum region with maximum at A=0'470yLt) about 

 3.10 -7 cm. 3 , which with the said cross section would mean 

 a length of 3.10 6 //. or over six million wave-lengths. But 

 this by the way only. The important thing is to see whether 

 the above numerical value of the average cross section of 

 blue light darts will continue to fulfil its function with regard 

 to the remaining "steps" (weaker and weaker exposures) 

 of plates coated with the same emulsion, the above being the 

 highest " step." These have just been completed in this 

 laboratory, ceteris paribus with the above one, and are now 

 being subjected to counts and area measurements. This 

 material will also serve to test the constancy of k/ N if, 

 varying n and a', their product is kept constant. 



An account of the results of these and of several other 

 experiments now in progress will be given in future papers. 



I gladly take this opportunity to express my best thanks 

 to Dr. C. E. K. Mees for having proposed to me the problem 

 of " discriminating, if possible, between the consequences of 

 a discrete and a continuous exposure theory/'' and to my col- 

 leagues Trivelli and Righter for furnishing the results of 

 their experiments. 



Rochester, N.Y.. 



January 19, 1922. 



Phil. Mag. S. 6. Vol. 44. No. 259. July 1922. T 



