and Laboratory Methods. 



1351 



brighter center could be determined within one 

 degree. 



It was found that the signal was obtained for all 

 angles of incidence precisely as if it was due to a 

 reflection from some interior surface, and it might 

 be assumed that the diffusion was due to this sur- 

 face not being perfectly smooth. In order to find 

 the angle between the supposed interior surface of 

 reflection and the surface of the section, 



Let O be center of revolution, 



Let C be collimator. 



Let T be telescope. 



Let N be normal to plate when yielding ordi- 

 nary signal, 



Let N^ be normal to plate when yielding 

 schiller signal. 



If COTr=2 cp then CON^TON^^y and, see Fig., a'=(^— e and b'=<7>+e. 

 But as the rays from C and to T must have been refracted, the true incident and 

 reflected rays for the internal surface cannot have been these, but rather such 

 rays as C O and T' O, making angles a and b with the surface normal O N', in 



.... sin a' ... sin b' , . , . , ^ ^ 



which sm a= and sm b= , // being the mean index of refraction 



for the moonstone. The normal to the internal surface will therefore be O N", 

 and the angle between the two surfaces will be equal to N'O N", denoted by d, 



and, from the figure, b — d^a-f-d, or d= 



b— a 



For Ceylon moonstone cut parallel ^oi d=12° 5' and 65° 19', and is 

 essentially independent of the angle of incidence. There is also a trans7nitted 

 image the color of which is complementary, that is, yellowish to reddish orange. 

 This would indicate that the violet and blue rays were diffused and reflected, 

 while the red, yellow, etc., penetrate. If the phenomenon were one of interfer- 

 ence, then if for a certain angle of incidence the reflected color is blue, it must 

 pass into red for a larger incident angle, and this it does not do. The theory of 

 internal reflection and absorption seems to best explain the phenomena. Similar 

 results were obtained with the Amelia Co., Va., albite and the adular of 

 Zillerthal. a. j. m. 



Thin lamellae with bright metallic lustre. 



Color on cleavage, silver white with 

 normal incidence, reddish brown with oblique incidence. G. 7.6 H. 1.5. Streak 

 lead gray. Ni2 TCg with traces Pb, Bi, and Au. 



Minerals of Japan. Kotora Jumbo, in Jour. 

 Coll. Sci. Tokio, 11: 213-281,1899. Nine 

 page Abs. Zeit. f. Kryst. 34: 215, igoi. 



Melonite. From Worturpa, South Australia. 

 Trans. Roy. Soc. S. Australia, 23 : 211, 1899. 



According to Vogt there should exist a 

 series of minerals, belonging to this 

 group, low in silica and containing 

 variable amounts of calcium and aluminium, varying between the limits of 



Fouque, F. Contribution a I'etude des miner- 

 aux de group de la melilite. Bull. Soc. Min. 

 23: ID, 1900. 



