REVERSED AND NON-REVERSED SPECTRA. 



157 



In figure 97, GG' and HH' are double slides like the carriage bed of a lathe, 

 each about 1.5 to 2 meters long and 10 cm. wide, rigidly fastened together. 

 They are placed at right angles to each other on a fiat table, the vacant distance 

 between G' and HH' being less than a meter. For ordinary purposes they need 

 not be screwed down. A, B, D, K, are flat carriages, or tables, provided with 

 screw sockets for supporting the different standards, and capable of sliding to 

 and fro with a minimum of friction. A carries the micrometer slit S. B and C 

 are joined by the Rowland rail R, whose length is thus equal to the radius of 

 the concave grating to be examined, or nearly so, so that the ends of R are on 

 vertical axes at b and d. B also supports the table C (somewhat enlarged in 

 the side elevation, fig. 98), on which the table t of the grating g may be adjusted 

 on its leveling screws. To secure a common axis, 6, e, the rod at ace is twice 

 bent at right angles. Moreover, if c is turned to one side, the supporting rod e 

 may be screwed into the vacant socket b at the end of R. For the case of fig- 

 ure 98, the angle of diffraction 6 is varied and \ = D sin 6, where D is the grat- 

 ing space. For the other case (c being turned aside and C screwed into and 

 turning with b) the angle of incidence is varied and X = D sin i. This is much 

 simpler in form than the early method used. 



97 



Finally, the table C carries the essentially new addition to the apparatus 

 (shown in front elevation in fig. 99), viz, the long slot ff, adapted to support 

 the right-angled reflecting prism E and at the same time to allow free play to 

 the rail R within ./f. Figure 99 then shows the progress of the rays (turned 90 

 to the front in a horizontal plane) from the slit or collimator, 5. They are 

 doubly reflected at E, return in a vertical plane and then impinge on the grat- 

 ing at G. The rays thereafter pass along the rail R (fig. 97) and are examined 

 by a strong eyepiece at d (not shown), rigidly but adjustably attached to the 

 near end of the rail. 



The displacement of K along HH ' is accurately measurable on a parallel 

 scale with vernier (not shown) . If Xi and x z are the two symmetrical readings 

 on opposite sides of the virtual slit image at S (fig. 97), and R the radius of 

 the concave grating, and x = x z x\ 



sn 



= x/zR, or sin i = x/z 



