1895J MICROSCOPICAL JOURNAL 165 



line and quinoline of refractive index 2.2, and the refrac- 

 tive index of the phenyl sulphide kindly supplied by 

 Prof. Klein's colleague, Prof. Erail Fischer, is only 1.56 

 instead of 1.95. If the dangerously poisonous and in- 

 flammable liquids are excluded, the list consists chiefly 

 of oils, the well-known Thoulet solution, monobrom- 

 napthalene, and methylene iodide. The solution of io- 

 dine in the latter frequently renders it insufficiently 

 transparent for the purpose. 



The determination of the true angle, 2V, between the 

 optic axes within the crystal, supposing it to be biaxial, 

 can at once be determined with the aid of the new ap- 

 paratus, by immersing the crystal in a liquid whose re- 

 fractive index is equal to the beta (intermediate) refrac- 

 tive index of the crystal. The condensing system of 

 lenses is first inserted between the polarizing nicol and 

 the stage, and the Bertrand lens above the analyzer ; as 

 objective, either the ordinary wide angle combination 

 usually employed for convergent light work, or a spe- 

 cially constructed one supplied for the particular pur- 

 pose of convergent light observations through an im- 

 mersion liquid is employed. This objective is so consti- 

 tuted that as large a field of vision as possible is afforded, 

 while the distance between the objective and crystal is 

 considerably greater than with the ordinary system in 

 use. The apparent angle of the optic axes in air, 2E, 

 may first be measured, if desired, after adjustment of 

 the crystal by means of the adjusting movements, by 

 bringing the hyperbolic brushes to the cross wire of the 

 microscope eye-piece in the usual manner. The immer- 

 sion cell not being in position while this is being achieved, 

 the objective can be approached nearer to the crystal and 

 one of the ordinary forms of convergent light objective 

 employed, which affords a larger angle of vision, reserv- 

 ing the special objective for the determination of the 



