CRYSTALLOGRAPHY MINERALOGY. 317 



crystal in parallel pencils crossing in it at all possible angles > 

 rays of the extreme outer pencils meeting at a very high angle 

 near the centre of the crystal section. On the emergence of the 

 light, it is brought back by the upper series of lenses into the 

 requisite conditions for the interference rings to be viewed by 

 the aid of a lens of low power, between which and the eye the 

 analysing Nicol is placed. 



And between the lens and the Nicol can be conveniently 

 placed a \ undulation plate, or a section of quartz equatorially 

 cut, when needed for the discrimination of positive and negative ' 

 characters in uniaxal and biaxal crystals. 



Instrumental additions to the polarising microscope extend its 

 use to the purposes of a stauroscope, the lenses then being sepa- 

 rated so as only to employ the central and approximately parallel 

 rays ; or again, where the angle of divergence is not too high, a 

 small graduated disc, as in the instrument of Soleil, allows of 

 measuring the angle between the optic axes. For the latter pur- 

 pose, however, it is preferable to employ a special instrument, 

 such as the goniometer of Babinet or the improved instrument 

 constructed for the purpose by V. von Lang, which leaves nothing 

 to be desired for accuracy and convenience. The description of 

 the methods of observation requisite for determining the character 

 of the dispersion of the optic axes by the polarising microscope 

 does not fall under our purpose here ; and as regards the instru- 

 ments requisite for determining the refractive indices in crystals, 

 though such instruments must be in the hands of the crystal- 

 lographer, and may even take the form of adjuncts to his goni- 

 ometer, the consideration of them really belongs to the more 

 general subject of physical optics. 



It will furthermore be sufficient here to make mention of the 

 instruments designed by Haidinger, by Groth, and others, in 

 which all or several of the processes of crystallographic optics 

 that have been alluded to are combined. The dichroscope of 

 Haidinger is an instructive implement for the study of the 



