172 



BARUS— ADJUSTMENT FOR PLANE GRATING [April 24, 



4. Data for Single Lens in Front of Grating. — In conclusion we 

 select a few results taken at random from the notes. 



The real test is to be sought in the coresponding values of 2x 

 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 measurement. 



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 other respects the conditions are similar 

 but reversed. Apart from signs, for the stationary grating 



,.2 ^2 



a' =■ a 





and for the rotating grating 



a" = a-^ -^. 



r^ — x^ 



The correction for shift loses the factor ih/f- 



i) and becomes 





- x^jr'- Vn^ - x^j 



As intimated, it is negative for the rotating grating and positive for 

 the stationary grating. It is eliminated in the mean values. 



6. Data. Single Lens Behind the Grating. — An example of 

 the results will suffice. Different parts of the spectrum require 

 focusing. 



Grating. Line. 



Stationarj- D2 



Rotating D2 



