AERANGEMENT OF APPARATUS. 



131 



easily made, and their completely excluding actinic 

 light. 



FIG. 28. 



Whichever method is adopted, before centring 

 with the low power objective, we may substitute the 

 dry front of the J, if it possesses one, for the top 

 combination and pin-hole cap of the French con- 

 denser. It can be easily made to fit it, if too large, 

 by a card-board ring slipping on the screw of the 

 French lens; or if too small by placing it in a card- 



K2 



