PROCEEDINGK OF THE SOCIETY. 247 



substances occur either as aggregates or in the colloidal state, a light 

 cone was visible, even if particles were not. When the solution referred 

 to was examined under the ultra-Microscope, nothing was to be seen 

 for some minutes, and then particles of sulphur became visible, which 

 increased in number and size until there was a brilliantly illuminated 

 cone of sulphur particles scintillating in the beam. It appears probable, 

 therefore, that there was a latent period lietween the mixing of the 

 solutions and the appearance of the sulphur particles. 



(2) He then discussed the artificial bleaching of green fiuorite by 

 ultra-violet rays. Pure white fiuorite was very difficult to obtain ; still 

 more so pure white fiuorite transparent to the extreme ultra-violet. A 

 green variety of fiuorite, however, was plentiful, and by exposing this 

 to ultra-violet light, such as that from the quartz mercury vapour lamp, 

 it was possible to bleach it perfectly in a very short time, and so obtain 

 white fiuorite. On passing a beam of light through fiuorite, he demon- 

 strated a beautiful violet-blue Tindall cone with the ultra-Microscope. 

 This cone arose from the ultra-violet light, which was transmitted and 

 changed in wave-length. There was a narrow band of ultra-violet 

 between the extreme end of the visible violet and the limit of trans- 

 mission of glass. By illuminating fiuorite with an ordinary beam, 

 through a screen consisting of a solution of sulphate of quinine (itself 

 transparent and colourless), the whole of the ultra-violet was cut out, 

 and the violet light cone disappeared. The ultra-]\Iicroscope showed 

 no differentiation of structure between pieces of fiuorite naturally colour- 

 less and others bleached by exposure to ultra-violet light, and it seemed 

 reasonable to anticipate that fiuorite bleached by ultra-violet light 

 would be as useful for making apochromatic objectives as natural white 

 fiuorite. 



The Secretary gave notice of the subjects which had so far been 

 arranged for discussion at the Meetings during the Session, and the 

 Meeting terminated. 



