CHAPTEK III. 

 OPTICAL APPARATUS. 



MOST of our readers are pretty sure to possess a microscope, 

 some objectives, eye-pieces, a substage condenser and a bull's- 

 eye. (NOTE : The term condenser shall hereafter be used as 

 applying solely to the " substage condenser;" while " bull's-eye" 

 shall be used to cover all condensing or parallelizing instru- 

 ments used between condenser and light). Some kind of so- 

 called microscopic lamp is also likely to form part of the out- 

 fit. As a rule a microscopic lamp not made specially for 

 microscopy is for various reasons unsuited to our purpose. 

 Such adjuncts to a microscope as a Polariser, a Paraboloid, a 

 Spot lens or other arrangement for " black ground illumina- 

 tion " shall be treated separately ; till further notice we shall 

 treat of axial transmitted light illumination only. 



The Microscope. The qualities essential to a microscope- 

 stand for our purpose are perfect rigidity, accurate working 

 of all parts, specially racks, fine adjustments, stage mechan- 

 ism, and draw tubes, if any are to be used. The substage ar- 

 rangements as to focusing and centering are just as important 

 as the other parts of the instrument. The writer proposes to 

 lay great stress on the accurate use of the condenser, which is 

 often treated in a careless and ignorant way, and sometimes 

 even by certain authorities omitted altogether. When the func- 

 tions of a condenser properly used are explained, it will be 

 seen how serious an omission this is. All the best photo- 

 micrographic apparatus we have ever seen, and all of which we 

 have seen illustrations that gave any promise of excellence, 

 have been used on the horizontal ; on the other hand there are 

 cases where the vertical position of the apparatus, i. e., the 

 horizontal position of the stage, is necessary. No microscope- 

 stand will meet both conditions unless it swing on a pivot, 



