4912.] 



FOR A CONCAVE GRATING. 



559 



platform for the camera or eyepiece, and a support for the slit. 

 These pieces may rotate independently^ ° and the camera platform 

 ■may be clamped to either bar. 



The linkage is supported at its intersection by blocks which slide 

 along the track. The arms of one of the parallelograms should be 

 adjustable in length so that the framework may be exactly set to the 



be 



:gratmg 



and so that gratings of slightly different radius may 



Fig. 4. 



Aised on the same stand. The grating slider is moved by a rod or 

 screw running to the end of the track near the slit, the only function 

 -of the linkage being to produce the necessary rotation. There is 

 itherefore no great stress on the axes tending to bend them. 



"Rotation of the slit is convenient in making the adjustments. 



