259 



(2) By the Specific Gravity bottle. 



Results 2-579 Mean. 2-582 



2-584 _ 



Refractive Indices of the Felspar. 



No refractometer being available indirect methods were used 

 and approximate results obtained. 



The presence of the acid oligoclase furnished an easy and 

 accurate qualitative test, by employing Becke's bright line 

 method. 



The felspar under consideration was found to have a lower 

 index than that of the oligoclase 



For ordinary differences, however, this gives no idea of the 

 amount of difference, but we may say that the values are less 

 than the following : — 



1-542 1-538 1-532 Oligoclase Levy. 



For exact determinations the principle was extended. 

 Although the amount of movement cannot be taken as a 

 basis of calculation in such an experiment, if we obtain two 

 substances which give no movement, we may assume that they 

 are of the same refractive index. 



Two sections were then taken, one parallel to (001) and one to 

 (OlO), and a portion of the cover-glass removed from each. The 

 balsam was then removed, first by turpentine and then by alcohol. 

 A drop of Klein's solution was placed in contact with the 

 mineral, and Becke's method applied, retaining the polariser, and 

 placing the section with its directions of vibration successively 

 parallel to the principal plane of the polariser. 



The solutions employed were, by trial and error, adjusted so 

 that their refractive indices were identical with those of the 

 mineral in its different positions. 



In this way four solutions were obtained. The refractive 

 indices of these were measured by means of a spectrometer. 

 Great difficulty was experienced in making these measurements 

 on account of the very strong absorptive power of the liquid. 

 Sodium, potassium, and strontium lights were tried, but of these 

 only potassium gave any results at all. Then the lights from 

 incandescent gases — hydrogen, nitrogen, oxygen, helium, and 

 argon and chlorine — were experimented with with small success. 

 Finally the illumination was obtained by means of an electric 

 spark between terminals composed of an alloy consisting of 

 zinc, cadmium, mercury, and tin in molecular proportions, pre- 

 pared by Sir William Crookes. 



In this way the minimum refractive index (a approximately) is 

 found to be 1-513 ; the maximum (y approximately) 1-525, but 

 from the difficulties of measurement the results are only roughly 

 correct. 



