Rowland? s Method for the Concave Grating. 



87 



or all but tight (in another method, stationary grating), so that 

 the table tt, which carries the grating g, may be fixed while 

 the nipple N" swivels in the T. Any ordinary laboratory 

 clamp Zand a similar one on the upright c (screwed into the 

 carriage S) secures a small rod k for this purpose. Again, a 

 hole may be drilled through the standards at K and e and 

 provided with set screws to fix a horizontal rod k or check. 

 The rod k should be long enough to similarly fix the standard 

 on the slide S carrying the slit and be prolonged further 

 toward the rear to carry the flame or Geissler tube apparatus. 



Fig. 2. 



Fig. 2. Elevation of the grating (g) and the eye-piece (E) standards. 



The table tt is revoluble on a brass rod fitting within the gas 

 pipe, which has been slotted across so that the conical nut M 

 may hold it firmly. The axis passes through the middle of the 

 grating, which is fastened centrally to the table tt with the 

 usual tripod adjustment. } 



2. Single Focusing Lens in Front of Grating. — I shall 

 describe three methods in succession, beginning with the first. 

 Here a large lens, Z, of about 56 cm focal distance and about 

 10 cm in diameter, is placed just in front of the grating, 

 properly screened and throwing an image of the slit S upon 

 the cross-hairs of the eye-piece F, the line of sight of which is 

 always parallel to the rod ao, the end b swivelled in the car- 

 riage C, as stated (see fig. 2). An ordinary lens of 5 to 10 cm 



