PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 785th Meeting was held at the Cosmos Club, March 3, 1917. 

 Vice President Humphreys in the chair; 49 persons present. The 

 minutes of the 784th meeting were read in abstract and approved. 



By invitation, Mr. A. H. Pfund presented an illustrated communica- 

 tion on, The colors of mother-of-pearl. At the request of the U. S. Bureau 

 of Fisheries the following work was undertaken to ascertain the cause 

 of the iridescence of mother-of-pearl. While this subject has been 

 studied before, the results have been largely of a qualitative nature. 

 By extending the observations into the infra-red region of the spectrum 

 where conditions are much simplified, the author has succeeded in 

 obtaining quantitative measurements which substantiate the explana- 

 tion of the colors of mother-of-pearl as given by Sir David Brewster. 



Under the microscope, a thin section of the shell of a fresh-water 

 mussel is seen to consist of three layers — the inner one being the true 

 mother-of-pearl (nacreous matter). This, in turn, consists of innumer- 

 able thin layers of CaC0 3 separated by extremely thin layers of organic 

 matter. As the shell grows, successive layers of nacreous matter are 

 deposited on the inside of the shell — each layer projecting slightly be- 

 yond the terminal edge of its predecessor. Hence the outside of the 

 nacreous layer will present closely spaced steps or ridges running parallel 

 to the growing edge of the shell. Such a surface structure is identical 

 with that of a diffraction grating, and hence diffraction colors ought to 

 exist. By laying bare the outer surface of the nacreous matter and 

 depositing an opaque layer of silver on it, striking diffraction colors are 

 seen. This surface structure may be studied most readily by means of 

 celluloid casts or replicas of the surface. These replicas show brilliant 

 diffraction colors. Under the microscope the individual lines may be 

 seen and the spacing may be determined. While this is subject to wide 

 variations, the range of spacing observed on the outside of the shell 

 of a fresh-water mussel ranges from 6000 to 25,000 lines per inch. As 

 might be expected, no trace of such a grating-like structure is obtained 

 from the inside of the shell. 



As a rule, the diffraction colors are rather inconspicuous, being 

 masked by the blaze of colored light from underlying regions. Since 

 this light is due to reflections from the numerous laminae and since 

 these are parallel and sensibly equi-distant, coloration due to inter- 

 ference might be expected. According to the usual theory, the optical 



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