Fig. 11. Sept. 14 and 18, 1916. Telephotographs of S. grotto at mght, looking 

 down from S.E. rim, bench on left, lake on right (see fig. 5a extreme left). Depres- 

 sion 3.26 ft. (99 m.). Arrows on bench point to flames (blue-green variety') which 

 were rising through recesses among spatter lumps. Dallmeyer lens of 11 'i'^'" diam- 

 eter. 43''" focal length, working at F 3'S (Series B). Wratten " M " plates. 



(a) Deep blue filter Wratten C. 24, aperture F 4, two seconds, 8 p. m. Sept. 14. 

 Shows stalaotite drip. Grotto flaming, blue rays. 



{b) Bright green filter Wratten B, aperture F 5, one second, 7 p. m., Sej)t. 18. Note 

 incandescent sjilashes on bench, and jagged edge of crust on lake, from w^hich there 

 was inrush to grotto. Grotto flamiug, green rays. Visually the continuous spec- 

 trum of the melt shows much green, but almost no blue. Hence the rendering of 

 the lines and si^lashes in contrast to (a). 



(c) No filter, aperture F 3-8. 1/50 sec, Sept. 18, 8 p. m. Crusts downsucking with 

 multiple fountaining raigi'atory to the left toward grotto. 



(d) No filter, aj^erture F 3*8, 1/^50 sec, Sept. 18, 9 p. m. Hexagon of crust being 

 engulfed at grotto. Note in c and d graded incandescence to centers of gas oxida- 

 tion. 



