348 



THE AMERICAN MONTHLY 



[Dec, 



removable iris diaphragm or any substituted apparatus. The 

 lower arm in this specific instance is provided with a revolving 

 plate G, upon which there is a sliding plate F, moved by rack 

 and pinion H, to receive the iris diaphragm and impart rotation 

 and excentric movements in the usual ways. The iris diaph- 

 ragm E, is provided around its inner periphery with a short 

 flange nicely fitting within the central opening in the sliding 

 plate F, and on the bottom just outside the flange with a pin, 

 slightly shorter than the flange, which drops into acorrespond- 

 hole in the sliding plate F and secures the diaphragm in proper 

 position. This pin may be duplicated opposite to render assur- 



ance doubly sure. In operation the iris diaphragm is simply 

 lifted up from its seat in the sliding plate and withdrawn en- 

 tirely from the sub-stage, leaving its position free to be filled, 

 for example, with the revolving apparatus shown in Fig. 2, 

 wherein the sliding plate F is indicated in central section hold- 

 ing the revolving receiver N, and this in its turn holding another, 

 M. In practice, N is slipped into place, and then M into its 

 place ; when either or both can be revolved in the same or oppo- 

 site directions ; in removal, M is first lifted up and withdrawn, 

 and then N. It is obvious that various selenites, mica plates, 

 diaphragms, etc., can be used in these revolvers very easily and 

 advantageously. Here is. for one thing, a complete and practi- 



