PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE PHILOSOPHICAL SOCIETY OF WASHINGTON 



The 763d meeting was held on November 27, 1915, at the Cosmos 

 Club. President Eichelberger in the chair, 42 persons present. The 

 minutes of the 762d meeting were read in abstract and approved. 



Mr. E. D. Tillyer presented a paper entitled A spectrograph for 

 photographing E talon rings. A systematic determination of the wave- 

 lengths of a spectrum such as the iron arc by means of the Etalon in- 

 terference rings requires a spectrograph giving sharp definition along 

 the spectrum lines and not necessarily sharply defined lines. The 

 spectrograph described was intended to be used in the ultra-violet 

 region from about 0.220 ju to 0.320 /jl and it was desired to obtain the 

 best possible definition through this region. A true flattening of the 

 field being impossible because of the absence of necessary materials 

 it was decided to use only quartz and rock salt in the optical system 

 and to so proportion the relative powers as to produce a flat though 

 inclined field when used with a 60° rock-salt prism. After setting up 

 the optical equations a solution was obtained which gave the neces- 

 sary flatness of field when the collimator was an ordinary quartz-rock- 

 salt objective achromatised to reunite X = 0.220 fj, to X = 0.320 /j. and 

 the camera lens was composed of two quartz lenses close together and 

 having almost normal field curvatures. The maximum curvature 

 in the hundred millimeters of field is less than a millimeter and could 

 have been further reduced except for the uncertainty of the indices 

 of the materials in the ultra-violet region. In practice this spectro- 

 graph will give fairly good definition throughout the visible spectrum 

 by a change of adjustment as well as in the region for which it was 

 designed. 



Mr. H. E. Merwin then spoke on Linear interpolation of wave- 

 lengths in spectrograms. That the curve for the spacing of lines on a 

 spectrogram is of the same form as a dispersion curve is shown thus: 

 Let i = angle of incidence on prism, A = angle of prism, n = refractive 

 index of prism, /3 = angle between photographic plate and normal to 

 back face of prism, d = distance from line on plate to normal to face 

 of prism; A, B, etc., are constants. Then d = (A/sin /3) (s/n 2 — sinH 

 — sin i. cot A), or, approximately, d = A (Bn — C) or A (Dn 2 = F) or d 



= (Gn-H) or (Kn 2 — L) . But the dispersion formula is n 2 = a+r^— — rfX. 2 



\~— c 



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