482 



Profs. Liveing and Dewar. 



[Apr. 3 r 



The paper concludes with some notes by Professor Mask elyne, on the 

 connexion between molecular phosphorescence and crystalline structure. 



The crystals experimented on have been the diamond, emerald, beryl, 

 sapphire, ruby, quartz, phenakite, tinstone, hyacinth (zircon), tour- 

 maline, andalusite, enstatite, minerals of the augite class, apatite, 

 topaz, chrysoberyl, peridot, garnet, and boracite. Of these, the 

 only crystals which give out light are diamond, ruby, emerald,, 

 sapphire, tinstone, and hyacinth. The light from emerald is crimson, 

 and is polarised, apparently completely, in a plane perpendicular to the 

 axis. Sapphire gives out a bluish-grey and a red light polarised in 

 a plane perpendicular to the axis. The ruby light exhibits no marked 

 distinction in the plane of its polarisation. 



Among positive crystals tinstone glows with a fine yellow light,, 

 polarised in a plane parallel to the axis of the crystal. So far the experi- 

 ments accord with the quicker vibrations being those called into play r 

 and therefore in a negative crystal the extraordinary, and in a positive 

 crystal the ordinary, is the ray evoked. Hyacinth, however, intro- 

 duces a new phenomenon, being dichroic, the colours, in three different 

 crystals, being pale pink and lavender — blue, pale blue and deep violet, 

 and yellow and deep violet-blue, polarised in opposite planes. 



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. 



VI. "Note on a Direct Vision Spectroscope after Thollon's 

 Plan, adapted to Laboratory use, and capable of giving 

 exact Measurements." By G. D. Liveing, M.A., Professor 

 of Chemistry, and J. Dewar, M.A., F.R.S., Jacksonian Pro- 

 fessor, University of Cambridge. Received April 3, 1879. 



Having seen in the "Journal de Physique" for May, 1878. the 

 account of M. Thollon's ingenious direct vision spectroscope, it 

 occurred to us that by a little modification we could adapt his plan so 

 as to produce an instrument well fitted for the work in which we 

 were engaged, combining the advantage of excellent definition, which 

 his plan secures, with the means of getting exact measurements with 

 the least possible chance of errors of adjustment or inequalities in the 

 working of the automatic system. The principle consists in having 

 two prisms only (half prisms as M. Thollon calls them), of which one 

 is fixed, and receives the light from the collimator by a reflecting 



