the Refractive Index and Dispersion of Glass. 



419 



Eight separate determinations were made by the immersion- 

 fluid method with the following results : — 



Number of 

 experiment. 



Observed ra D for 

 Thoulet solution, 



151493 

 1-5 14'. 12 

 1-51495 

 1-51488 



1-51491 

 1-51489 



1-51494 

 1-51491 



Difference in fifth 

 place from mean value. 



1 



2 



+ 1 

 



+ '6 

 -4 

 -1 

 — 3 



-1 



3 



4.. 



(> 





8 





Mean value 1-51492. 



Average difference from mean value ='00002. 

 Value obtained for the index of the prism by a direct 

 reading on the refract ometer =1-51490. 



H 



An inspection of: the results obtained shows that the 

 indices of the glass prism and the surrounding liquid c;in be 

 equalized to an average accuracy of 4- "00002. The accuracy 

 with which the indices may be equalized is clearly pro- 

 portional to the angle of the prism, or to the angle between 

 the cathetal surfaces if the glass examined is not in the form 

 of a regular prism. In the present instance this angle was 

 about 10°, and an even higher order of repetition accuracy is 

 to be expected in the case of an irregularly shaped piece of 

 glass in which the surfaces of the refracting edge chosen 

 meet at an angle greater than this. It is evident, then, 

 that the " Schlieren-methode " is well adapted for the 

 equalizing of the indices to one or two units in the fifth 

 place, and is therefore all that is required in commercial 

 practice. 



It was found that when the straight edge F had been 

 moved into coincidence with the image of the edge B, and 

 even after it had overlapped it slightly, the field could not 

 be obtained absolutely dark, and a small image of the 

 extended light source A persisted in remaining visible in 

 the field of view of the observing telescope. This was 

 eventually traced down to an out-of-focus flare-spot image 



2F 2 



