88 C. Bar us and M. Burn* — Plane Grating Similar to 



focal distance, with an appropriate diaphragm, is adequate and 

 in many ways preferable to stronger eye-pieces. The slit 8, 

 carried on its own slide and capable of being clamped to c 

 when necessary, as stated, is additionally provided with a long 

 rod hh lying underneath the carriage, so that the slit 8 may be 

 put accurately in focus by the observer at G. F\& a carriage 

 for the mirror or the flame or other source of liijht whose 



Figs. 3, 4, 5. 



cfca. 3 



Figs. 3, 4, 5. Diagrams. 



spectrum is to be examined ; or the source may be adjustable 

 on the rear of the rod by which D and 8 are locked together. 



Finally the slide AB is provided with a scale ss and the 

 position of the carriage C read off by aid of the vernier v. A 

 good wooden scale graduated in centimeters happened to be 

 available, the vernier reading to within one millimeter. For 

 more accurate work a brass scale in millimeters with an 

 appropriate vernier should of course be used. 



Eye-piece E, slit 8, flame F, etc., may be raised and lowered 

 by the split tube device shown as at M and M' in fig. 2. 



