92 C. B<( nix taxi M. Barns — Plane Grating Similar to 



In this method the slide S and D are clamped at the focal 

 distance apart, so that flame, etc., slit, collimator lens and grat- 

 ing move together. The grating may or may not be revoluble 

 with the lens L on the axis a. 



8. Data for the Collimator Method. — The following data 

 chosen at random may be discussed. The results were obtained 

 at different times and under different conditions. The grating 

 nominally contained about 15,050 lines per inch. The efficient 

 rod length ah was R = 169-4 cm . Hence if 1 / C= 15,050 X 

 ■3937x338-8, the wave-length \ = C-2x"'\ 



Grating Lines 2x' %x 



Stationary _. 2> 2 118-30 118-19 



Rotating D 2 118-08 118-19 



Stationary _Z> 2 118-27 118-16 



Rotating Z> 2 118-05 118-16 



Rowland's value of Z> 2 is 58-92 X 10 -6 cm ; the mean of the 

 two values of 2x just stated will give 58-87XlO _8cm . The dif- 

 ference may be due either to the assumed grating space, or to 

 the value of R inserted, neither of which were reliable abso- 

 lutely to much within -1 per cent. 



Curious enough an apparent shift effect remains in the 

 values of 2a? for stationary and rotating grating, as if the colli- 

 mation were imperfect. The reason for this is not clear, 

 though it must in any case be eliminated in the mean result. 

 Possibly the friction involved in the simultaneous motion of 

 three slides is not negligible and may leave the system under 

 slight strain equivalent to a small lateral shift of the slit. 



9. Discussion. — The chief discrepancy is the difference of 

 values for 2x in the single lens system (for D, z , 118-7 and 

 118-5 cm , respectively) as compared with a double lens system 

 (for D„ 118-2 cm ) amounting to -2 to "4 per cent. For any 

 given method this difference is consistently maintained. It 

 does not, therefore, seem to be mere chance. The detailed 

 investigation, which must be omitted here,* made it clear that 

 the effect of focusing is without influence on the diffraction 

 angle and much within the limits of observation. It is, there- 

 fore, probable that the residual discrepancy in the three 

 methods is referable to a lateral motion of the slit itself due to 

 insufficient symmetry of the slides AA and BB in the above 

 adjustment. This agrees, moreover, with the residual shift 

 observed in the case of parallel rays in § 8. 



10. Reflecting Grating. — The adjustment of the plane grat- 

 ing if cut on specular metal is nearly identical to the above, 

 except that the collimator is fixed as a whole in front of the 

 grating, either to the slide carrying the standard of the grating, 

 B, or else quite in front of the cross slide A A, fig. 5 above, 



*See Proc. Am. Phil. Soc, 1. e. 



