3o6 



University of California. 



[Vol. i. 



A value for the mean index of refraction §— 1.671 for sodium 



light was obtained by substitution in the formula 6 — n '—. — ~> 



sin V a 



where n = 1.6053 the index of refraction of cassia oil. 



By the usual method with specially-cut prisms, the following 

 values were obtained for the three indices of refraction for sodium 

 1 ight : — 



a — 1.6650 

 /i = 1 .6690 

 y = 1.6840 



from which y — a = .019 and - + — + - = 1.6726. 



3 



Before suitable material was obtained from which to cut prisms, 

 the method of the Due de Chaulnes was employed to determine 

 a and y for white light. The focusing was done on a large model 

 Nachet microscope, using an attenuated wash of carmine as a 

 signal for the bottom of the thin plate. With the greatest care 

 this method is subject to the possibility of serious error, but the 

 final results obtained agree fairly well with those already given for 

 sodium light. They are: — 



a = I.659 

 y = 1.685 



A thin plate cut parallel with the axial plane, and showing 

 between crossed nicols red of the second order, was investigated 

 by means of the comparateur of Michel-Levy, and the value 

 y — a = . 0207 obtained for the measure of the double refraction 

 in white light. This agrees as closely as could be expected with 

 the value arrived at with the prisms for sodium light. 



The dispersion of lawsonite is p < v. 



In certain rather thick basal sections a strong pleochroism was 

 observed — blue parallel with a and colorless, or with a slight tinge 

 of yellow, parallel with 6; the absorption being a>6. The formula 

 for the absorption of the mineral is 0>6 = C. The color is very 

 unevenly distributed, however, and in some crystals may be entirely 

 lacking. It is generally arranged in zones, or in narrow bands par- 

 allel with the prism, and therefore parallel with the prismatic cleav- 



