NICHOLS— PHOSPHORESCENCE OF SULPHIDES. 



263 



Taking the relative frequencies, i. e., reciprocals of the approxi- 

 mate wave-lengths (i//a X lo^), of the crests as estimated from the 

 curve, it is found that the intervals are either very nearly 58 or 

 twice that number. If a series having 58 as its constant interval 

 be formed with one member located at the principal crest (A = 4800) 



Fig. 3. 



other members of this series will coincide with the subordinate crests 

 of the curve. The short vertical lines in Fig. 3 indicate the posi- 

 tions of those members of such a series as coincide with the crests 

 and of two further members which fall on a slight and not very 

 well defined maxima at .5562 /^ and .5921 ix. 



