THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 029 



C. Observations extending to other crystals of the same 

 mineral and other minerals. 



7. Determine the thickness of the rock-slice and calculate 

 the birefringence of the mineral under examination (p. 625). 



8. By determining the character of other sections of the same 

 mineral or of those showing special features, determine that of 

 the mineral itself (pp. 620-624). 



9. Determine the refractive index of the mineral as far as 

 circumstances permit (pp. 625-628). 



EXPLAXATIOXS OF FIGURES. 



Fig. 1. Crystal section in the position in which it is originally 

 drawn with index-reading zero. Most of the details 

 shown are subsequently added. The directions of 

 extinction are represented by thick lines, crystallo- 

 graphic directions by interrupted lines, and the posi- 

 tion of the cross wires only by short thick lines. 



,, 2. Crystal section with the long edge and cleavage parallel 

 to the right and left cross wire. The index-reading 

 83 is shown on the right end. The directions which 

 originally coincided with the cross wires are shown 

 by short thick lines. The actual cross wires are re- 

 presented here and in fig. 3 by thin continuous lines. 



,, 3. Crystal section with the fast ( ) direction of vibra- 

 tion parallel to the slot. The quartz wedge is inserted, 

 and shows a black band where it exactly neutralises 

 the crystal. The smaller figures in the scale on the 

 wedge indicate hundreds, the larger thousands of 

 micro-millimetres of relative retardation. 



,, -i. Double quartz wedge. 



,, 5. Combination gypsum plate and quartz wedge. 



,, 6. Mica steps. 



7. Straight central isogyre showing only one branch. 

 The inner arrows give the directions in which the 

 ends of the isogyres would move if the stage were 

 rotated in the direction of the outer arrows. 



,, 8. The same with gypsum plate inserted. The lower 

 colours on the farther side of the isogyre show that 

 the character of the section is fast ( ). 



