THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 621 



In a biaxial crystal the character of the greater number of 

 sections is the same as that of the mineral, which is that of its 

 acute bisectrix. The smaller this angle the more frequently 

 the character of sections coincides with that of the mineral. 



A section with pendulum movement and higher relative retarda- 

 tion than most other sections of the same mineral with the same 

 thickness will always have the same character as the mineral itself. 



A pseudosymmetric section indicates that the crystal is neutral, 

 in other words, that its optic axial angle is 90, but only certain 

 sections of a neutral crystal are pseudosymmetric, and other 

 neutral sections do not necessarily belong to a neutral crystal. 



The character of the longitudinal direction is best ascertained 

 in the object-image in the manner already described, but as will 

 be seen it may also be determined from the directions-image 

 itself. 



To identify the longitudinal direction and its proximal and 

 distal ends in the sketch of the object-image, the stage should 

 be rotated till in the directions-image the central isogyre is seen 

 to coincide or make an angle of less than 45 with the right and 

 left cross-wire and have its proximal end to the right. This 

 may be termed the index position of the isogyre, for the index 

 reading will then give the position of the proximal end of the 

 longitudinal direction of vibration. 



When the isogyre is in the index-position, the character of the 

 longitudinal direction may be determined by inserting a gypsum 

 plate in the slot (figs. 8 and 16). If this be done the isogyre itself 

 will assume the colour characteristic of the plate, but will be 

 bordered by higher colours on one side and lower colours on the 

 other. If the colours are lower on the same side as the proximal 

 end of the slot, the character of the longitudinal direction and 

 therefore of the isogyre and section will be the same as that of the 

 direction of the gypsum plate parallel to the slot. By the 

 proximal end of the slot is meant of course that nearest the 

 proximal end of the isogyre and of the longitudinal direction of 

 vibration. If the procedure already described has been fol- 

 lowed, this will be the right-hand end. If the colours are higher 

 on the same side as the proximal end of the slot, the character 

 of the section will be opposite to that of the direction in the 

 gypsum plate parallel to the slot. 



For instance, if the slot be in the position indicated in figs. 8 



