106 WRIGHT: MEASUREMENT OF REFRACTIVE INDICES 



index for oblique waves transmitted in the vertical plane nor- 

 mal to the principal section (column 5) ; but a noticeably higher 

 refractive index than the liquid for light waves incident under 

 conditions of column 6, and a much lower refractive index for 

 conditions of column 7; also that the refractive index of a basal 

 section of calcite will appear, if observed under the condi- 

 tions of obhque illumination of column 2, to have a lower re- 

 fractive index than the maximum refractive index co; under simi- 

 lar conditions (column 4) a section parallel to the principal 

 axis will appear to have a higher refractive index than the mini- 

 mum refractive index e which it might be expected to show. It 

 may be noted, however, that, even under the most favorable 

 conditions, the rays incident on the mineral grain are not all 

 contained in the diametral plane normal to the principal section. 

 Of the cone of rays emerging from the condenser all but a small 

 segment is eliminated by the sliding-stop ; the marginal rays of 

 this segment include an appreciable angle with the diametral 

 plane, and are accordingly transmitted through the mineral grain 

 under conditions different from those in the diametral plane; the 

 phenomena which we observe are the result of the aggregate 

 action of all the rays and not solely of those of the diametral 

 plane, on which the determinations are based. This defect can 

 be remedied to a large extent by the use either of small circular 

 holes^ or of a narrow slit opening in the sliding-stop. Disregard 

 of this factOi"'fnay affect the accuracy of measurements on strongly 

 birefractirig I niinerals. These relations serve also to explain 

 the long-e'6tttinued appearance of colored fringes on the margins 

 of such crystal grains when observed in white light, these fringes 

 persisting over a much greater range of refractive indices than 

 might be expected were they due solely to the difference in dis- 

 persion between mineral and liquid. 



In central illumination the inclination of the transmitted 

 rays is, in general, less than that in obhque illumination with 

 the result that the error introduced because of this factor is 

 proportionately less. Offsetting this advantage in part, however, 

 is the sensitiveness of central illumination to the character, 



'Tscherm. Miner, petrogr. Mitteil., 20:240. 1901. 



