348 



THE AMERICAN MONTHLY 



[Dec, 



removable iris diaplirajim or any Mibstituted apparatus. The 

 lower arm in this specific instance is provided with a revolving 

 plate G, ii])on which there is a slidinj; plate F, moved hy rack 

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

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

 ra^m E, is provided around its inner periphery with a short 

 fiance nicely fitting within the central opening in the sliding 

 plate F, and on the bottom just outside the llan^'e with a pin, 

 slightly shorter than the llange, which drops into acorrespond- 

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

 ]>osition. This ))in mav be duplicated opposite to render assur- 



ance doubly sure. In ojieration tiie iiis diapliragm i.s simply 

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

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

 for example, with the revolving apjtaratus 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 i^lace, 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 site directions; in removal, M is first lifted up and withdrawn, 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 tiiat various selenites, mica plates, 

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

 advantageously. Here is. for one thing, a com|»lete and practi- 



