THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 627 



If the directions of vibration of adjoining crystal sections 

 are parallel, exact comparison of the refractive indices corres- 

 ponding to each pair of parallel directions may be carried out 

 in the same manner. In other cases parallel nicols should be 

 employed and the stage rotated till their direction of vibration 

 bisects the angle between the directions of the pairs of vibration 

 the refractive indices of which are to be compared. In all 

 cases a comparison of the mean refractive indices may be made 

 by dispensing with the use of a nicol. 



If a crystal or grain be immersed in Canada balsam or other 

 medium, such as a highly refracting liquid, or a larger crystal, 

 the relation between its refractive index and that of the sur- 

 rounding material may be determined by the Schroder van der 

 Kolk or " finger ' method. A condenser is employed and 

 placed close below the object, or the effects will be reversed. 

 One side of the illumination is then shaded, usually by the 

 finger placed below the lower nicol. If a shadow appear on 

 the same side of the object as the ringer is placed, the 

 refractive index of the object is higher than that of the 

 medium. If it appear on the opposite side, the refractive 

 index is lower than that of the medium. By means of a nicol 

 the two directions of vibration can be separately examined in the 

 manner already explained. 



With monochromatic light this method gives good results. 

 It is usual to provide a series of liquids, the refractive indices 

 of which differ by small amounts, starting from about 1*47 

 and extending up to 1*76, afforded by methylene iodide, or 1*83 

 by a solution of sulphur in methylene iodide, which is, however, 

 not so satisfactory. If an exact determination be required, a 

 mixture of two liquids is prepared, which has as nearly as possible 

 the same refractive index as the mineral, and the index of refractive 

 of this mixture is determined by the Abbe refractometer. It is 

 important to remember that the refractive indices of liquids 

 change considerably with the temperature. 



If white light be employed, the phenomena are complicated 

 by the fact that the dispersion of the colours in liquids is usually 

 much greater than with solids of the same refractive index, and 

 a series of colour phenomena may result, which complicates the 

 observation. In the case of minerals with decidedly higher or 

 lower refractive indices than the medium there is no difficulty ; 



