53 



rays of light, after falling on the mirror, converge 

 to the focal point, which in this case is at the upper 

 end of the tube (Fig. 38). The photographic plate 

 may be fixed in the axis of the tube or placed at 

 one side, where it receives the image after reflec- 

 tion from a plane mirror mounted diagonally. In 

 another arrangement a convex mirror takes the 

 place of the diagonal plane and sends the rays back 

 toward the base of the tube, where they are again 

 reflected by a plane mirror, either to a point on 

 one side of the tube or down through the hollow 

 polar axis, to unite in an image on the slit of a 

 powerful spectrograph mounted in a constant- 

 temperature chamber. 



Fir,. 39. 6o-inch Reflector, showing Stellar Spectrograph on Tube. 



