1891] MARYLAND ACADEMY OF SCIENCES. 141 



remberg's Polarizer. The tliiiiner sections were quite 

 blue; but those of a thickness of one-sixteenth of an inch, 

 gave in the polarized field a black cross with a system of 

 rainbow rings beginning with blue in the centre sur- 

 rounded by a yellow, then a red, then a pale green ring 

 which was lost in the margin. 



I was surprised at the difference presented between 

 what I saw and the usual whitish ground with a black 

 cross and rings tinged a pale yellow. Referring to Tait 

 (Light) I found that Dispersion has been accounted for 

 by the different speeds of light of different wave-lengths 

 in the same refracting medium; — this being a consequence 

 of ultimate grained structure of ordinary matter which is 

 on a scale not incomparably smaller than average wave- 

 length. But the colored rings themselves remained. 



At this juncture I prepared and sent specimens of the 

 find to Dr. D. T. Day, Prof. C. S. Hastings, Dr. Steeg 

 of Hamburg, and Dr. Williams of the Johns Hopkins 

 University. A short time thereafter a letter was brought 

 to me from Dr. Steeg pointing out that the Apophyllite I 

 sent him possessed a dispersion quite different from Aj^o- 

 phyllite found in other places. The ring system of this 

 Apophyllite is similar to that of the Subsulphate of 

 Strontium, and is optically negative while all other Apo- 

 phyllites are positive. This indication can be beautifully 

 demonstrated by means of the Calkspar-wedge, "C No. 

 47 of our Catalogue." Calcite is termed the negative 

 crystal because the extraordinary ray is less refracted 

 than the other, or the axial elasticity is the greatest; 

 while the other class are termed positive (Lewis Wright). 



Apophyllite, says Wright, is remarkable for the fact 

 that it is "positive" for one end of the Spectrum, "nega- 

 tive" for the other, and non-doubly refracting for some 

 intermediate color, generally yellow. Hence we have 

 instead of rainbow circles, as in most cases, rings nearly 

 white and black: the usual effect being black rings lined, as 



