PLATE XVII. 



Tho difforence in lovol betwcoii I'hito XI and I'luto XII is too sroat to permit 

 of a satisfactory study of the changes in form of tlie flocculi at different heights 

 above tlie ])liotos])here. In the present series it is fortunately p()ssil)le to give 

 an intermediate step. ol)tained by setting the second slit immediately outside of 

 II.,; the level shown therefore lies between that of Plate XV and that of Plate 



xViii. 



PLATE XVIII. 



This ph(itogra|)h is given here in tlu' endeavor to bring out the bright 

 eruptive tongues, hardly to be distinguished friiUi the less lirilliant tlocculi. 

 The abnormally dark background n(>cessarily results from the deep i)rinting 

 re(iuired to show the exceedingly brilliant details. 



162 



