2i8 SCIENCE PROGRESS. 



the slide is thrown slightly out of focus by raising the tube, 

 and a bright border then appears along the line of junction 

 of the two minerals, on the side of the more refringent. 

 Now a given section of a felspar has two refractive indices, 

 say a and y, which can be separately investigated by using 

 the polarising Nicol, and if the section has a birefringence 

 not too low, these indices will not differ much from a and 

 y, the least and greatest indices of the mineral. Similarly, 

 a section of quartz has two refractive indices, w and e', the 

 former being constant, while by a suitable choice we can 

 be sure that e' does not differ much from s. If we can find 

 a favourable contact of felspar and quartz, which extinguish 

 nearly in the same position between crossed Nicols, we 

 know that either the axes of a and w approximately coincide 

 and those of y and £ (parallel position), or a and £ coincide 

 and also y and w (crossed position). The two cases can be 

 distinguished by the use of a quartz-wedge or similar con- 

 trivance. In the former case we can test, by the method 

 already indicated, whether a is greater or less than w, 

 and whether y is greater or less than e. In the latter 

 case we can compare o with e and y with to. If we can 

 find crystals realising each of the two cases, we thus 

 obtain four data. By reference to the known refractive 

 indices of different varieties of felspar, Becke thus divides 

 these minerals into six groups, which can be discriminated 

 in this way. 



It will be seen that this method is only adapted to 

 felspars associated in the same rock with quartz (20), and 

 that its application depends upon the occurrence of sections 

 favourably oriented. Further, it does not distinguish be- 

 tween the labradorites and more basic varieties. Michel 

 Levy (16) has, however, extended and generalised Becke's 

 comparison of refractive indices. His plan is, briefly, to 

 compare the mineral to be investigated with a highly 

 refracting liquid. To do this the cover-glass is removed 

 from the slice, the mineral cleaned, and a drop of the liquid 

 brought in contact with it. It can then be found which 

 has the higher refractive index. He uses for the purpose 

 Klein's heavy solution, diluted to have a suitable refractive 



