1909-] SIMILAR TO ROWLAND'S METHOD. 173 



The values of 2x, remembering that a centimeter scale was used, 

 are again surprisingly good. The shift is computed by the above 

 equation. It may be eliminated in the mean of the two methods. 

 The lens L' may be more easily and firmly fixed than L. 



7. Collimator Method. — The objection to the above single-lens 

 methods is the fact that the whole spectrum is not in sharp focus at 

 once. Their advantage is the simplicity of the means employed. If 

 a lens at L' and at L are used together, the former as a collimator 

 (achromatic) and with a focal distance of about 50 cm., and the 

 latter (focal distance to be large, say 150 cm.) as the objective of 

 a telescope, all the above difficulties disappear and the magnification 

 may be made even excessively large. The w'hole spectrum is bril- 

 liantly in focus at once and the corrections for the shift of fines 

 due to the plates of the grating vanish. Both methods for stationary 

 and rotating gratings give identical results. The adjustments are 

 easy and certain, for with sunlight (or lamplight in the dark) the 

 image of the slit may be reflected back from the plate of the grating 

 on the plane of the slit itself, while at the same time the transmitted 

 image may be equally sharply adjusted on the focal plane of the 

 eye-piece. It is therefore merely necessary to place the plane of 

 spectra horizontal. Clearly a' and a" are all infinite. 



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

 tance apart, so that flame, etc., slit, collimator lens and grating 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 con- 

 tained about 15,050 lines per inch. The efficient rod length ah was 

 i?= 169.4 cm. Hence if i/C= 15,050 X -3937 X 338.8, the wave- 

 length X=C 2x cm. 



Grating. Lines. 



Stationary Z?- 



Rotating D2 



Stationarj- D2 



Rotating D2 



