BULL'S EYE CONDENSER. LO\V POWERS. 225 



are therefore, when essaying the use of the Beck, com- 

 pelled to fall back on a larger and heavier stand.* 



BULL'S EYE CONDENSER. 



This instrument, so well known, and accompanying 

 almost every stand as sold, we make nearly constant use 

 of. It should be simply a large plano-convex lens, say 

 from two to three inches in diameter, and fitted with 

 universal mountings. It has been too general a rule with 

 the microscope makers to "adapt" the size of the con- 

 denser to that of the stand. Thus, if a certain maker 

 furnish five or six different sizes of stands, he will be 

 pretty sure to have as many sizes of condensers to accom- 

 pany the same. But let the reader insist that, however 

 small may be the model of the stand selected, the con- 

 denser be at least two inches in diameter, and that even 

 three inches will be found none too large. Any of our 

 opticians can furnish the instrument made to order. 

 We have seen and worked with several made by Zent- 

 mayer to accompany his large and intermediate stands, 

 and also the large model furnished with the Messrs. 

 Beck, all of which were effective instruments. 



WORKING WITH LOW POWERS. 



It has been our primary intention throughout this 

 work to avoid repetition of instructions, hints or sug- 

 gestions, such as may be found in the various text- 

 books, while interspersed among the pages already writ- 



* The construction of the "Acme" admits the use of the Beck illumina- 

 tor. 



15 Microscopy. 



