116 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Optical Constants of the Schefperite. 

 By Dr. Gustave Melczer. 



The indices of refraction were determined on polished phites, prepared 

 hy Voigt & Hochgesang, by means of the Abbe total reflectometer and 

 with the application of the differential method proposed by Viola.* 

 There were used (1) a plate approximately perpendicular to the acute 

 bisectrix, (2) a thin section similarly oriented, and (3) and (4) two plates 

 parallel to the base (001). Since the plates were not transparent, the 

 boundaries of the total reflection could only be measured with the reduc- 

 ing telescope, but with this, especially in the case of the last two plates 

 (owing to their excellent polish), the limits could be fixed within 1 to 3 

 minutes. By reading every 15 degrees and in the vicinity of the maxima 

 and minima every 5 degrees (which according to my experience com- 

 pletely suffices with the reducing telescope), the boundary curve was 

 constructed, and the following maxima and minima determined for Na 

 light : 



(1) t 64° 261' and 64° 22' 62° 56' and 62^ 26' 



(2) ? ? 62° 57|' 62° 271' 



(3) 64° 25' 64° Ij-' 62° 55' 62° 23^' 



(4) 64° 20|' 64° 1' 62° 51|' 62° 23' 



The boundaries for the comparison prism were determined, after each 

 pyroxene determination, by five readings 90° apart, as follows : 



(1) 62° 11' (2) 62° loy (3) 62° llf (4) 62° 11^' 



The cause of these noticeable differences in w of the prism cannot 

 lie in the temperature, for this only differed by 3° on the different days 

 when the measurements were made, nor can it be due to lack of homo- 

 geneity in the glass hemisphere, for in using the enlarging telescope the 

 mean of two readings in diametrically opposite positions of the horizon- 

 tal circle were only 15 seconds apart. The cause can therefore only lie 

 in the use of the reducing telescope itself, the accuracy of which, even 

 with the sharpest boundaries, is only within 1 to li minutes, while with 

 the enlarging telescope the same boundaries can be determined within 10 

 to 15 seconds. 



* Zeit. fiir Krjst, XXX. 438 and XXXII. 313. 



