180 PHOTOMICROGRAPHY 



an ordinary substage condenser, is to place a metal or 

 cardboard stop under the condenser to block out the 

 central rays of light and to permit the passage of the 

 peripheral rays only. The size of this stop must be 

 adjusted both to the focal length of the condenser and 

 the N.A. of the objective, in order to cut out entirely the 

 dioptric beam and yet leave a sufficiently large hollow cone 

 to illuminate the object brilliantly. The stop must be large 

 enough to occlude an aperture slightly larger, say 10%, 

 than the N.A. of the objective, and its diameter is deter- 

 mined, according to a rule given by Dr. Spitta in Micro- 

 scopy, by multiplying the equivalent focus of the con- 

 denser by twice the N.A. to be stopped out. They are 



Fig. 52 



WHEEL STOP 



made in the shape shown in Fig. 52, and placed in 

 the ring beneath the condenser, or they can be made 

 with a central pin-point, over which discs of correct size 

 for the objective in use are placed. The Traviss expand- 



Fig. 53 



TRAVISS' EXPANDING STOP 



ing stop, Fig. 53, is very useful for the purpose, as the 

 size of the stop can be adjusted to a nicety by moving 



