RICHARDS AND ARCHIBALD. 



GROWING CRYSTALS. 



349 



Diameters of Successive Images. 



These all show greater growth in the first interval than in the second. 

 In order to reduce them to one standard, the diameter of the third ap- 

 pearance was taken in each case as unity. The table then becomes : — 



Diameters of Successive Images. 



At the time of the first appearance, the average age of the crystal 

 must be about half the time intervening between two exposures ; for the 

 crystal must have been formed since the last exposure, and it is as likely 

 to come near the beginning as near the end of the interval. Thus in 

 Figure 11 the crystals evidently started immediately after the previous 

 exposure, while in Figure 9 they were registered while still very young. 

 The averaging of a much larger number of observed diameters led to the 

 slightly different values given below, corresponding to the accompanying 

 times : — 



