MERWIN: chromatic reflection IX COVELLITE 343 



Absorption is so strong that the color of covelHte even in the 

 finest powder is due to Ught reflected directly from the surfaces 

 of grains. Yet differences in absorption of different colors are 

 not sufficient to cause marked differences in the reflection of 

 these colors. Therefore we look to the refractive index and 

 reflective power to explain the color effects akeady described. 



I have estimated that for light from orange to violet which enters 

 the surface at normal incidence about one-fourth is transmitted by a 

 plate 0.5 M thick. For these colors the absorption index, k, is of the 

 order of 0.1; for red it may have twice^ this value. Absorption has, 

 therefore, very little effect upon the reflective power of the mineral in 

 the visible spectrum. The reflecting power, R, represents the reflected 

 fraction of the normall}' incident light as given b}^ the equation 



nV.2 + (n + ly 



in which n is the refractive index for the wave-length having the ab- 

 sorption index (extinction coefficient) k. The following equations de- 

 fine k: 



— iir riKd —iTTKd 



I = Iie~^^~~ ; or / = I^e >^^~ (1) 



The intensity of the light entering the surface is 7i. After traversing 

 the distance d the intensity is /. The wave-length of the incident 

 light in the ether is X. The other symbols have their usual signifi- 

 cance. Xi, is the assumed wave-length within the absorbing medium. 

 If 7 and /i represent amplitudes of vibration then the exponent of e 



becomes — ^^- . These equations define k aqcording to the usage of 



A 



most of the recent writers. Yet in the foflowing phj^sical tables both 

 the name and the definition of this sjmibol vary. In Tobies Annuelles 

 Internationales, page 133, for 1912, names and definition are as above; 

 in Landolt, Bdrnstein and Roth, page 966 (1912) the symbol k, called 

 the absorption index, = nx above; in Smithsonian Physical Tables, page 

 195 (1914) first two equations, k, called the absorption index, = k. 

 Also in Wood's Physical Optics (1911) in the chapter on the optical 

 properties of metals a: is named and defined as above, but in the chapter 

 on the theory- of dispersion = nK above. 



For values oi n < 3 the reflective power increases much faster 

 than n. For w^hite light incident from air at an angle of 60° on 

 plates cut parallel to the cleavage about 20 per cent of the blue 



* Or several times, for W. W. Coblentz found very high reflecting power in 

 the near infra-red on an inclined section (Carnegie Inst. Wash. Pub. 97). 



