Volcano ôshima, Idzu. 



65 



mersed successively in liquids of different refractive indices and 

 the determination was made on each fragment whether the refrac- 

 tive index of the liquid, p, was above, below, or lay between, the 

 two values of the refractive indices of the fragment, n^ and ih. 

 Getting the results n^<nl<p' in any trial and j)"<n^'<n!' in 

 another, ,î was determined from (5) as p"<ß<2>'- 



In the writer's practical work with refractive liquids, Weight's 

 solutions were used for feldspars, and mixtures of monobroranaph- 

 thalene and methyleniodide for the mafic minerals. The indices 

 of refraction of these sets of liquid for sodium light were measured 

 on a refractometer and their constancy was checked from time 

 to time during experiments. 



Optical Orientation. — Optical orientation of feldspar was deter- 

 mined with the cleavage piece parallel to M (010) — in which one 

 of the optical binormals is visible through a conoscope — by the 

 following method : — 



Examining the section with a conoscope and putting the zero- 

 isogyre parallel to one of the vibration planes of the niçois, the 

 azimuth {<p) and the central angular distance (p) of the pole were 

 measured with the screw micrometer ocular, p being reduced from 



the observed linear distance first to 

 the apparent angle by means of Mal- 

 lard's formula and then to the true 

 angle within the crystal, using the 

 value of ß obtained by the method 

 described above. The position of 

 extinction was then observed ortho- 

 scopically, and all the data obtained 

 were plotted in stereographic projec- 

 tion as in Fi^. 34 where is the 



