36 



merwin: media of high refraction 



to 2.10, and to extend the series beyond 2.4Na, or in special cases, 

 particularly when a refractometer is not at hand for standard- 

 izing the liquids, take the place of media hitherto used. For the 

 latter purpose solids have been found which may be mixed in 

 given proportions by weight to produce permanent standard 

 media. These are either vitreous (more or less rigid) or liquid 

 (fluid). 



Altho differences in refractive index as small as 0.001 can be 

 detected under the microscope by either the method of oblique, 

 or of central illumination, it is seldom that in determinative 

 mineralogical work results closer than 0.01 are of practical use 

 owing to the complex character of most minerals. For this 

 reason many details concerning the preparation and use of these 



TABLE 1 



media are not included here but may be found in the complete 

 record of the work when published. 



Liquids, n = 1.74 io 1.87. In 100 parts of methylene iodide 

 at 20° the number of parts of the various substances indicated 

 in the table (1) can be dissolved, forming saturated solutions 

 having the permanent standard refractive indices specified. 



When ready for use, the liquids can be mixed by means of a 

 dropper to give intermediate refractions. Commercial iodoform 

 (CHI3) powder is not suitable, but crystals from a solution of 

 the powder in ether may be used, or the crystallized product 

 may be bought. A fragment of tin in the liquids containing 

 Snl4 will prevent discoloration. 



Liquids, n = 1.74 io 2.28. Near its boiling point methylene 



