370 0. Andersen — Aventurine Feldspar. 



light reflected from the opaque lamellae was always of the 

 same grayish color, whereas the reflections from the trans- 

 parent lamellae showed vivid colors varying with the absorp- 

 tion colors, that is, with the thickness of the lamellae. It was, 

 for instance, observed that the absorption colors of some 

 lamellae changed gradually from very light yellow near the 

 edges to light brown red towards the middle. The interfer- 

 ence colors in reflected light changed in the same lamellae 

 from dark gray of first order at the edges to the brilliant colors 

 of second order near the middle. 



In the thin lamellae of yellowish or light brown tints (in 

 transmitted light) the interference colors were not noticeably 

 modified by the absorption of the one of the interfering rays 

 that passed through the lamellae. As the lamellae became 

 thicker, however, the influence of the absorption was 

 more pronounced and in the deep brown red lamellae the 

 absorption was so strong as to suppress the interference colors. 

 The reflected light of these lamellae was therefore grayish like 

 that of the opaque lamellae. 



We know that the thickness of the lamellae is about 1/2 of 

 the path difference between the interfering rays reflected from 

 them. It is obvious, then, how the thickness of the thinner 

 lamellae that show distinct interference colors can be approx- 

 imately determined. The very thinnest of the lamellae showed 

 the interference colors gray and white of first order and their 

 thickness could accordingly be estimated at from 50 to 100 p.fi. 

 Thicker lamellae showed interference colors from yellow of 

 first to red of second order corresponding to thickness of from 

 150 to 500 yu/i. The majority of lamellae had a thickness of 

 between 100 and 400/*/^. In sections where the lamellae were 

 cut approximately perpendicular to their planes they appeared 

 as almost invisible streaks entirely too thin to be measured 

 with the ordinary devices of the microscope. 



During the observation of the interference colors it was 

 noticed that light reflected from lamellae with small angles p 

 never showed any detectable polarization. 



Absorption colors. 



The color of the reflecting lamellae in transmitted light 

 varied with their thickness from very light yellow and reddish 

 brown to deep brown red or blood red. 



Lamellae forming small angles p with the plane of the section 

 showed no pleochroism. Lamellae of large angles p, on the 

 other hand, appeared at first sight to be strongly pleochroic. 

 In ordinary thin sections of the feldspar we observe the follow- 

 ing absorption colors of such lamellae (e. g. lamellae parallel to 

 (112) seen in sections after (010) or lamellae parallel to (150) in 



