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-XII. Crossed Spectra obtained by Combinations of Different 

 Interferometers, and their Applications to the Measurement 

 of Difference in Wave-lengths. By H. Nagaoka, Professor 

 of Physics, and T. Takamine, Graduate in Physics, Lnperial 

 University, Tohjo *. 



[Plate I.] 



THE method of crossed spectra introduced by Kundt was 

 utilized hj Gehrcke and v. Baeyer f for discriminating 

 the ghosts by means of interfere rice points produced by 

 Lummer-Gehrcke plates placed at right angles to each other. 

 This effective method can be applied to combinations of: 

 different intereferometers, and the crossed spectra thus 

 obtained may sometimes be utilized for the purpose of 

 accurately determining the difference in wave-lengths. We 

 have studied some of these combinations in experiments 

 relating to the structure of mercury lines and their Zeeman 

 effect, and we believe that crossed spectra will be of some 

 interest to those engaged with spectroscopic work. 



The spectra produced by the combination of Lummer- 

 Grehrcke plates were treated by Gehrcke, v. Baeyer, and 

 • Janicki %, so that it will be unnecessary to discuss them here. 

 We tried crossing a plane grating with an echelon, an 

 echelon with another echelon, an echelon with a Lummer- 

 Gehrcke plate, a Lummer-Gehrcke plate with a Fabry-Perot 

 air-plate, and a Fabry-Perot air-plate with an echelon. The 

 ;first way of crossing was not effective on account of low 

 resolving power of the grating, the second, though somewhat 

 better, was not at all comparable with the advantages that 

 can be gained by the last three methods of combination ; on 

 this account we limit our discussion to these three kinds of 

 crossed spectra. 



Echelon Grating and Lummer-Gehrcke Plate combined. — 

 The echelon grating and the plane-parallel plate used in the 

 present experiment were made by Hilger, and had the 

 following dimensions and constants : — 



Echelon: number of plates 35, thickness of plate 9*350 

 .mm., step 1 mm., length 32*73 cm., 



SXmax. =580-5 m.A.U. for 5790 

 = 576-0 „ „ 5769. 



= 509-0 „ „ 5461. 



* Communicated by the Authors. 



t Gehrcke and v. Bae} r er, Ann. der Phys. xx. p. 269 (1906). 



X Janicki, Ann. der Phys. xxxxix. p. 439 (1912). 



