90 < '. Barus and .)/. Barm — Plum Grating Similar to 



It follows also that a'X<i" = <('. For a <= 80 cm and sodium 

 light, the adjustment showed roughly /' = 650 cm ,/" = 570, 

 the behavior being that of weak concave lenses. The same 

 a = 8Q m and sodium light showed furthermore a' = 91 and 

 a"==70-3. 



Finally there is a correction needed for the lateral shift of 

 rays, due to the fact that the grating film is enclosed between 

 two moderately thick plates of glass (total thickness £ = -99 c, ")of 

 the index of refraction n. Moreover, since this shift is on the 

 rear side of the lens L, its effect on the eye-piece beyond will 

 be (if/" is the principal focal distance and b the conjugate focal 

 distance between lens and eye-piece, remembering that the 

 shift-must be resolved parallel to the scale ss) 



*-*( * - 1 ■ \ ( b -l ), 



where the correction e is to be added to 2a?, and is positive for 

 the rotating grating and negative for the stationary grating. 



Hence in the mean values of 2a? for stationary and rotating 

 grating the effect of e is eliminated. For a given lens at a 

 fixed distance from the eye-piece {b/f—1) is constant. 



4. Data for Single Lens in Front of Grating. — In conclu- 

 sion we select a few results taken at random from the notes. 







Table I. 







Grating 



Line 



Observed 'Zx' 



Shift 



Corrected 2x 



Stationary 



G 



132-60 



— •26 



132-34 





V, 



118-90 



— •23 



118-67 





F 



98-23 



— •19 



98-04 





Hydrogen 



87-87 



— •16 



87-71 





Violet 









Rotating 







132-10 



+ •26 



132-36 





A 



118-45 



-23 



118-68 





F 



97-90 



•19 



98-09 





H. Violet 



87-50 



•16 



87-66 



The real test is to be sought in the corresponding values of 

 2a? for the stationary and rotating cases, and these are very 

 satisfactory, remembering that a centimeter scale on wood and 

 a vernier reading to millimeters only was used for measure- 

 ment. 



5. Single Focusing Lens Behind the Grating. — The lens 

 L', which should be achromatic, is placed in the standard 

 behind g. The light which passes through the grating is now 

 convergent, whereas it was divergent in §2. Hence the focal 

 points at distances a', a" lie in front of the grating; but in 



