METHODS OF PETROGRAPHICAL RESEARCH. 219 



index, and for testing this he makes use of fragments of 

 standard minerals having known refractive indices ; so that 

 in reality the liquid is used merely as a medium to institute 

 a comparison of refractive indices between the crystal 

 studied and known minerals. 



We have hitherto said little of methods involving the 

 use of convergent light. These have been regarded by 

 petrographers rather as affording confirmatory tests in the 

 identification of doubtful minerals than as being of sys- 

 tematic use. With Dick's polarising microscope, however, 

 the rapid and effective application of convergent light to 

 even minute crystals in rock-slices may often be of great 

 service ; for instance, for distinguishing quartz and felspar 

 in the fine-grained aggregates of some crystalline schists. 

 On the other hand, where larger crystals can be studied, 

 the instrument advocated by Fedorow (14) has introduced 

 a new element of precision into such measurements as those 

 of the angles between optic axes, etc. 



Several writers have pointed out how, when suitably 

 oriented sections occur in a rock-slice, such measurements 

 may in many cases be effected with parallel light. Lane, 

 for example (21), calculates the optic angle of a mineral 

 from comparison of the birefringence of sections parallel to 

 two of the principal planes. It is evident that such methods 

 can be of only occasional application. 



There are many ways in which chemical tests may be 

 used as an aid to the microscopical study both of rock-slices 

 (left uncovered for the purpose) and of powdered material. 

 Although for the isolation of minerals heavy solutions will 

 be preferred, when practicable, to acid solvents, the latter 

 may be profitably employed in particular cases, as, for 

 instance, in separating augite from a basaltic rock. Again, 

 observations of the relative rates at which different minerals 

 are attacked by hydrochloric or hydrofluoric acid are in- 

 structive. These may be carried out by etching the surface 

 of a rock-slice with the acid and stopping the process after a 

 certain time; or, if the powdered rock be used, a systematic 

 method of "fractional decomposition" may be followed, 

 successive applications of the acid being allowed to act for 



