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



and 16, and the colours be lower on the margin of the isogyre 

 farther from the observer as in those figures, the character of 

 the section will be the same as that of the plate parallel to the slot. 



These methods apply both to uniaxial and biaxial crystals. 

 The inferences that can be drawn from the characters of one or 

 more sections with reference to that of the mineral have already 

 been described (pp. 620, 621). 



Sections Perpendicular to an Optic Axis. These may be re- 

 cognised in the object-image by the darkness in all positions * 

 in a uniaxial mineral, and in a biaxial mineral by a feeble 

 illumination which does not vary when the stage is rotated. 

 They show no relative retardation and are therefore neutral. 



If a gypsum plate be inserted in the slot over the directions- 

 image of such a section of a uniaxial crystal, the black cross 

 in the directions-image will be represented by a cross of the 

 colour characteristic of the plate ; and if the mineral be of the 

 same character as the direction in the plate parallel to the slot, 

 the quadrants through which the slot passes will show higher 

 colours, and the other quadrants lower colours, while if it be of 

 the opposite character, the contrary will be the case. If a quartz 

 wedge or mica ladder possessing the same character as the 

 mineral be pushed progressively in, the rings of colour in the 

 former quadrants will contract, and in the latter will expand, and 

 if it possess the opposite character, the same phenomena will 

 occur in the alternate quadrants. 



In biaxial crystals cut at right angles to an optic axis, the 

 isogyre always passes through the centre, and has two distal ends. 

 In certain positions it is straight and parallel to a cross wire and 

 lies in the optic axial plane. If then the stage is rotated through 

 45 towards the slot, the isogyre becomes a branch of an hyperbola 

 with its axis parallel to the slot (fig. 19). If a gypsum plate be 

 now inserted, the hyperbolic isogyre will show the interference 

 colour of the plate, and at the same time, if the crystal 

 have the same character as the direction in the plate parallel 

 to the slot, the concave margin will exhibit higher colours 



* Isotropic crystals will also be dark in all positions in the object- 

 image, but the directions-image will show a uniformly dark field 

 instead of a cross, unless, as sometimes happens, the glass of the ob- 

 jective is in a state of strain, when a feeble uniaxial cross may be 

 visible. 



