IRIS DIAPHRAGMS. 



171 



Beck's Iris Diaphragm (fig. 102) is very simple, and 

 on that account preferred. By pressing the lever 

 handle placed at the side of the brass box the aperture 

 is gradually made to close up, and without for a 

 moment losing sight of the object. 



FlG. 102. Beck-Brown's 

 Iris Diaphragm, 



FIG. 102a. Collins- Davis' s Iris Nose- 

 piece Diaphragm. 



Collins's Limiting Diaphragm, or Aperture Shutter. 

 Fig. 102 shows the instrument as a nose-piece for screw- 

 ing on to the lower end of the microscope tube. This 

 form of aperture shutter enables the observer to adjust 

 his objective to any aperture he wishes, and the closing 

 of the shutter does not contract the absolute size of 

 the field, but limits its brightness ; in this way the 

 true value of penetration is observed without moving 

 the eye from the tube. 



Mr. Nelson suggests the application of a series 

 of diaphragms in connection with an ingenious cen- 

 tring nose-piece devised 

 likewise as a sub-stage. 

 This piece of apparatus 

 is recommended as a use- 

 ful addition, and as a 

 convenient and inexpen- 

 sive centring sub- stage 

 for small instruments. 

 The optical part of a T 4 o 

 objective forming the 

 condenser, and which for 

 the purpose should be 

 fitted with the shortest 



possible adapter, so that the diaphragms may be brought 

 close to the back lens. The sub-stage is seen in fig. 103. 



FIG. 103. Nelson's Sub-stage Condenser. 



