proceedings: philosophical society 237 



retardation of two beams reflected from two successive layers is 2/xe cos r 

 where e is the thickness of the layer, /j. the refractive index and r the 

 angle of incidence. If this retardation is a whole number of wave- 

 lengths, constructive interference will take place. thence maxima of 

 intensity will exist when 



2/j.e cos r = Xi, 2X2, 3X 3 

 etc., where X2 = -^ an d X3 = -5- 



Such maxima were looked for by reflecting white light from the 

 polished surface of nacreous matter and by measuring the intensities 

 in the various spectral regions by means of a spectrometer and vacuum 

 thermo-couple. Since the interference maxima are most widely sepa- 

 rated and are sharpest in the infra-red, the observations were carried 

 out in this spectral region. Results obtained from brilliantly colored 

 specimens were entirely in agreement with the above theory, i.e., the 

 wave-lengths of the interference maxima were in the ratio 1:|: £. 

 This quantitative determination proves definitely that the brilliant 

 iridescence of mother-of-pearl is due to interference. 



By determining the refractive index of the nacreous matter, it was 

 found possible to determine the actual thickness of the laminae. The 

 results, which differed somewhat for different specimens, showed that 

 the thickness is of the order 0.4-0. Q/j.. 



Discussion. The paper was discussed by Messrs. Sosman and 

 Wright. Mr. Pfund was of the opinion that the number of layers 

 of material in the shells examined was too great to correspond with the 

 number of days or with the number of tides occurring during the life 

 of the shell. 



Mr. W. F. Meggers then presented an illustrated paper on Inter- 

 ference measurements of wave-lengths, and infra-red spectrum photog- 

 raphy. Interference measurements of wave-lengths in the portion 

 of the iron arc spectrum in which the International Secondary standards 

 exist (3233 A to 6750 A) have been made at the Bureau of Standards. 

 The wave-lengths of about 320 lines were compared with those of the 



85 secondary standards by the interferometer method of Fabry and 

 Perot. The average interval between standards was thus reduced 

 from 40 A to 8 A, making accurate interpolation easier. Whereas 



86 per cent of the secondary standards have intensities 4 to 6 inclusive, 

 only 47 per cent of the new lines have these intensities, the remainder 

 being distributed among fainter and stronger lines. This makes it 

 possible to photograph sufficient standards with a wider latitude of 

 exposure. 



Similar measurements were made in the spectra of the rare gases, 

 helium, neon, argon, krypton, and xenon. The wave-lengths were 

 compared with the fundamental spectroscopic standard (wave-length 

 of red radiation from cadmium = 6438.4696 A). The measurements 

 in the neon spectrum show differences in the numbers of waves per 



