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



of a thin section in a rock-slice only relative determinations of 

 refractive indices are possible in an ordinary petrological micro- 

 scope, comparison being made either with the Canada balsam 

 or other medium in which the rock-slice is immersed, or with 

 an adjoining crystal. 



For the Becke method a high power is employed, and the 

 cone of illumination is narrowed. This may be effected by 

 removing or lowering the condenser and inserting a card- 

 board slip with a hole one or two millimetres in diameter 

 twenty or thirty millimetres below the stage.* A slit of the 

 same diameter parallel to the boundary of the section may be 

 substituted. This is equally effective and does not cut down the 

 light to the same extent. The boundary surface between the 

 section and the medium or adjoining crystal must be at right 

 angles to the surface of the rock-slice. This can be verified bv 

 observing if it remains constant in position when the focus is 

 varied. 



If now there be an appreciable difference between the re- 

 fractive indices on opposite sides of the boundary, one margin 

 of the boundary will usually be seen to be lighter than the field 

 in general and the other darker. If the objective be focused 

 on a point in the neighbourhood of the upper surface of the 

 section, the light margin of the boundary will be on the side 

 with the higher refractive index and the dark margin on that 

 with the lower refractive index. If the focus be gradually lowered, 

 these bands will be reversed in position. 



In this way it is possible to determine whether the refractive 

 index of a crystal mounted in Canada balsam is higher or lower 

 than that of this substance. If, however, the crystal is un- 

 covered, and its margin free from balsam, it may be immersed 

 in a succession of films of liquid of different refractive indices 

 and its refractive index thus determined between comparatively 

 narrow limits. 



If the crystal section be birefringent the observation should 

 be made with the lower nicol in position, and first one and then 

 the other direction of vibration in the crystal should be brought 

 into parallelism with the direction of vibration in the lower nicol. 

 In this way the indices of refraction parallel to both directions 

 of vibration may be determined. 



* An iris diaphragm is often provided, and is more convenient. 



