obtained by Combinations of Different Interferometers. 129 



with that given by the Lummer-Gehrcke plate, the confusion 

 as to orders o£ spectra and the uncertainty as to the presence 

 of ghosts can be easily avoided. Experiments in this 

 direction are now in preparation. Crossed spectra obtained 

 by the combination of a concave grating with a Fabry- 

 Perot interferometer were utilized by Eversheim * for the 



Fi*. 1. 



O + iT 

 ^ +0.6 

 & + *$ 





+ 105 \ n %« 



Ex \ \-liO ~ 



r:2 3 



Echelon and Lummer-Gehrcke Plate. 



determination of wave-lengths, and by Wood f for the study 

 of the band spectrum. 



Plane-parallel Plate and Air-Plate combined. — The Fabry- 

 Perot interferometer used in the present experiment was 

 made by Hilger, and was of the sliding type ; the air-plate 

 had the range of 12 cm., one of the plates being movable by 

 a screw, which was provided with a side micrometer for 

 slow motion. The guide for displacing a half-silvered glass 

 plate was nearly perfect, which is an essential quality for 

 the work now under discussion. 



Light from a monochromator passed through the air-plate 

 and fell on a prism attached at one end of the plate for 

 direct vision. By turning the plate so that the light fell at 

 proper incidence, the interference points could be made to 



* Eversheim, Ann. der Phys. xxx. p. 815 (1909). 

 f Wood, Phil. Mag. xxvi. p. 176 (1913). 



Phil. Mag. S. 6. Vol. 27. No. 157. Jan. 1914. K 



