320 SCIENTIFIC APPARATUS. 



different systems of crystals, or of other important physical features. 

 Such would be models of the wave surface and of the isothermal 

 ellipsoid in crystals, or in illustration of the different varieties of 

 dispersion. It would, however, be unprofitable to discuss here 

 the various forms which the needs of the lecture-room and the 

 ingenuity of the lecturer might take, in illustrating these subjects. 



But one most valuable educational implement remains to be 

 alluded to, namely, the use of collections, typical or otherwise, 

 illustrative of Mineralogy as a classificatory science, or of crystals, 

 natural and artificial, selected with knowledge and discrimination 

 to illustrate salient facts, and the more interesting and exceptional 

 features of Crystallography. Among such illustrative collections 

 would be classed crystal sections cut so as to exhibit the varieties 

 of divergence of the optic axes or of their dispersion, or the influ- 

 ence of heat, strain, &c., on their optical elasticity, or exhibiting 

 the isothermal curves illustrating their conductivity; apparatus 

 and crystals to represent the phenomena of pleiochroism, or the 

 remarkable relations that connect pyro-electric characters, or 

 rotatory polarisation, with peculiarities in the symmetry of the 

 crystals exhibiting these properties. 



And for the study of Petrology, a science that can only be pro- 

 perly pursued by a complete mineralogist, microscopic sections of 

 minerals occurring in rocksj and of the different kinds of rocks in 

 which they occur, should be associated with typical collections 

 illustrating either a general series or particular groups of rocks, or 

 those of special localities. And it will be very desirable to see 

 presented before the eye the best lapidaries' implements and 

 accessories to them, for cutting, polishing, and mounting micro- 

 scopic sections. Good and not necessarily expensive forms of 

 microscope, with means of ready adjustment for the use of polar- 

 ised light, and of effecting measurements by its aid, and such 

 additions perhaps as may render the spectroscope applicable for 

 simple observations, will also find their place in a complete 

 exhibition of mineralogical and petrological apparatus. 



NEVIL S.-MASKELYNE. 



