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CONTENTS. 



(A) 

 VOL. 210. 



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I. A Wave-Lew ftli Comparator for Standards of Length : An Instrument for Fine 



Measurement in Wave- Lengths of Liyltt. Witli an Appendix on the Use of 

 Wave-Length Rulings as Defining Lines <>n Standards of Lenytli. By 

 A. E. H. TUTTOX, M.A., D.Sc., F.R.S page 1 



II. The Elastic Limits of Iron and Steel under Cyclical Variations of Stress. By 



LEONARD BAIRSTOW, A.R.C.Se., Wh.Sch. Communicated by Dr. R. T. GLAZE- 

 BROOK, F.R.S. 35 



III. A Critical Study of Spectral Series. Part 1. The Alkalies H and He. By 



W. M. HICKS, Sc.D., F.R.S. 57 







IV. The Diffraction of Electric Wares Hound a Perfectly Reflecting Obstacle. By 



H. M. MACDOXALD, F.R.S., Professor of Mathematics in the University of 

 Aberdeen 113 



V. The Initial Accelerated Motion of Electrified Systems of Finite Extent, and the 



Reaction Produced by the Resulting Radiation. By GEORGE W. WALKER, 

 A.R.C.Se., M.A., Superintendent of Eskdalemuir Magnetic Observatory. 



Communicated by Sir JOSEPH L ARMOR, Sec.R.S. 145 



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