662 
MR. W. CROOKES ON MOLECULAR PHYSICS IN HIGH VACUA. 
play, and therefore in a negative crystal the extraordinary and in a positive crystal 
the ordinary is the ray evoked. 
“ A crystal of hyacinth, however, introduced a new phenomenon. In this optically 
positive crystal the ordinary ray was of a pale pink hue, the extraordinary of a very 
beautiful lavender-blue colour. In another crystal, like the former from Expailly, the 
ordinary ray was of a pale blue, the extraordinary of a deep violet. A large crystal 
from Ceylon gave the ordinary ray of a yellow colour, the extraordinary ray of a deep 
violet-blue. 
“ Several other substances were experimented on, including some that are remark¬ 
able for optical properties, among which were tourmaline, andalusite, enstatite, 
minerals of the augite class, apatite, topaz, chrysoberyl, peridot, garnets of various 
kinds, and parisite. So far, however, these minerals have given no result, and it will 
be seen that the crystals which have thus far given out light in any remarkable 
degree are, besides diamond, uniaxal crystals (an anomaly not likely to be sustained 
by further experiment); and the only conclusion arrived at is, that the rays whose 
direction of vibration corresponds to the direction of maximum optical elasticity in 
the crystal are always originated where any light is given out. As yet, however, the 
induction on which so remarkable a principle is suggested cannot be considered 
sufficiently extended to justify that principle being accepted as other than probable.” 
