1902.] Refractive Indices of Fluor ite, Quart?:, and Calcite. 335 



very inferior to Nelson's, as the indices can soon be calculated, and it 

 can easily be turned into simple arithmetic without the drawing. But 

 the indices here given have all been observed. There has been no 

 attempt to make them fall into line by any kind of correction, such as 

 the use of a freehand curve or by interpolation. 



10. Right and Left Quartz. — Some doubt having arisen as to the 

 indices of right-handed quartz being the same as those of left-handed, 

 I have had prepared by Mr. Hilger a prism of left-handed quartz of the 

 same dimensions as the prism of right-handed quartz used in the fore- 

 going measurements. I have with this prism determined the indices, 

 ordinary and extraordinary, for the mean D line, and have repeated 

 the corresponding observation with the right-handed prism used in the 

 previous determinations, as before referred to. The results are as 

 follows : — 



Ordinary. Extraordinary. 



March 4, 1900. Right quartz 1-5442558 1'5533662 



December 27, 1901. Right quartz... 1*5442558 1-5533673 



January 9, 1902. Left quartz 1-5442363 1-5533452 



Difference 0-0000195 0-0000221 



The value given for left quartz and the ordinary ray by Van cler 

 Willigen (for wave-length 5895-37, which he names D) is 1*54417. 



Finally a rough determination of the specific gravities was made as 

 follows : — 



Right quartz. Left quartz. 



2-6495 2-6511 



It will thus be seen that while the refractive indices of the right- 

 handed quartz are greater than those of the left-handed, the specific 

 gravity is less. 



11. Results of the Measurements. — The results of the observations are 

 given in the following table of refractive indices : — 



