624 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



In neutral crystals and in those in which the optic axial angle 

 differs but slightly from a right angle, the optic normal, as we 

 have seen, shows no cross, and those seen in sections at right 

 angles to the two bisectrics are indistinguishable. 



If the optic axial angle is so small that both optic axes are 

 visible in the same section, the methods already described for the 

 case where one optic axis is present may be employed (fig 14). 



Variations of Relative Retardation in a Directions-Image. There 

 is usually a decrease in the relative retardation indicated by 

 the interference colours towards the proximal margin or margins 

 of the field, and this may be utilised to determine the position 

 of the longitudinal direction and its proximal end. For this 

 purpose the stage is rotated though 45 from the position of 

 extinction. Unless an optic axis is visible, the isogyre will 

 then have passed out of the field and the region of lowest 

 interference colours will mark the position of the proximal end 

 of the longitudinal direction (fig. 12). If the section be at right 

 angles to a line of optical symmetry, there will -be two opposite 

 regions of lowest relative retardation (fig. 15), and the line joining 

 them will be the longitudinal direction. This method is fre- 

 quently useful where the isogyre is indistinct.* 



In doubtful cases the gypsum plate may be inserted, and then 

 the region that approximates most closely to the colour of the 

 plate will have the lowest birefringence and indicate the prox- 

 imal end and longitudinal direction. If this be at a point within 

 45 of the slot and the colour be higher than that of the plate, 

 the character of the section will be the same as the character 

 of the plate. If it be more than 45 from the slot, or the 

 colour be lower, the character will be opposite to that of 

 the plate. If both these conditions hold good, it will be the 

 same as that of the plate. In neutral sections the lowest 

 colour will be about 45 from the slot. 



If the optic axis be in the field, the line joining it to the 

 centre will be the longitudinal direction. Some portions of 

 the field will then have a higher and others a lower colour than 

 the plate, and the colour in the centre of the field will determine 

 the character of the section, unless of course the optic axis is in 

 the centre of the field, when the character will be neutral (p. 622). 



Theodolite Stage. In recent years Fedorov has introduced 

 * J. W. Evans, Mia. Mag., vol. xiv., 1907, pp. 233-4. 



