1894.] On Hollow Pyramidal Ice Crystals. 113 



sources are employed, the apertures may be exceedingly fine, and the 

 nionochromatism is of very high order. 



To diffuse the issuing light, a tube of 2 ins. diameter and equal 

 length, carrying within it either of two diffusing screens of ground 

 glass, of fine and extremely fine texture respectively, is attached to 

 the tapped annulus of the frame of the exit slit by a suitable carrier, 

 which enables the tube to be approached as near to the slit as desired 

 by sliding along a bar. The instrument to be illuminated, the polari- 

 seope of the axial angle goniometer for instance, is brought close up, 

 so that the end of the polarising tube enters the diffusing tube and 

 almost touches the ground glass screen, which is best distant about 

 Ijr ins. from the slit ; the axes of the optical tubes of the two in- 

 struments should of course be made continuous. The illumination 

 of the field of the polariscope, when carrying an adjusted crystal 

 section-plate between its convergent lens systems, is so bright that 

 measurements of the optic axial angle can be carried out with light 

 as far as G, and is greatly superior to that obtained by the use of 

 coloured flames. The interference figures are wonderfully sharp upon 

 a homogeneously coloured and illuminated background. 



Cases of crossed axial plane dispersion can be completely traced 

 from the extreme separation of the axes for red in one plane to their 

 extension for blue in the plane at right angles, and the exact wave- 

 length for the crossing point when the biaxial crystal simulates an 

 uniaxial one at once determined. 



The instrument is equally adapted for use in the determination of 

 refractive indices by the methods of refraction or total reflection ; the 

 refracted images of the slit of the spectrometer are immensely 

 brighter than when coloured flames or a hydrogen Geissler tube are 

 employed. A great saving of time is effected in all these measure- 

 ments, and this is especially advantageous in observations for dif- 

 ferent temperatures. Full details of the mode of employing the 

 instrument for these various observations, and for its use in stauro- 

 scopical determinations of extinction angles, are given in the memoir. 



V. "On Hollow Pyramidal Ice Crystals." By KARL GROSS- 

 MANN, M.D., F.R.C.S.E., and JOSEPH LOMAS, A.R.C.Sc. 

 Communicated by Professor JUDD, F.R.S. Received 



January 4, 1894. 



(Abstract.) 



I. The Lava Cavern, Surtshellir. At a visit to the lava cavern, 

 Surtshellir (Iceland), in June, 1892, the farthest recess, which con- 

 tains ice stalactites and an ice pond, was found to be covered on walls 

 and ceiling with ice crystals in the form of hollow hexagonal 



