1892.] 



THE MICKOSCOPE. 



Ill 



which is attached to the metal tubes, the terminal end of the out- 

 let tube being furnished with a small glass tube by means of 

 which a too 'rapid outflow of water is prevented. The tubes of 

 the freezing-box are so arranged as to prevent their revolving 

 with the revolutions of the table. In.B is shown a sectional view 

 of the freezins" arrangement. — Americafi Monthlv Microscopical 

 youriial^ February. i8g2. 



The Abbe Illuminator or Condenser. — For all powers, 

 but especiallv for high-power objectives, a condenser or illumi- 

 nator is of great advantage. The one most generally useful was 

 designed bv Abbe. It consists of two or three very large lenses 

 which are placed in some form of mounting beneath the stage. 

 It serves to concentrate a very wide pencil of light from the mir- 

 ror upon the object. For the best work in modern histology the 

 Abbe illuminator is almost as indispensable as the homogeneous 

 immersion objectives. 



20 20* . 2r 21 



Explanation of Plate. 



Fig. 20. 20a, 21, and 22. Sectional views of 

 the Abbe Illuminator of 1.20 N. A., showing 

 various methods of illumination. Fig. 20, 

 axial light with parallel rays. Fig. 20^, ob- 

 lique light. Fig. 21, axial light with con- 

 verging beam. Fig. 22, dark-ground illumi- 

 nation with a central stop diaphragm. 



Axis. The optic axis of the illuminator and 

 of the microscope. The illuminator is cen- 

 tred, that is its optic axis is a prolongation 

 of the optic axis of the microscope. 



S. Axis. Secondary axis. In oblique light 

 the central ray passes along a secondary axis 



of t^e illuminator, and is therefore oblique to 

 the principal axis. 



A, Fig. 2£ represents the upper part of the 

 illuminator. 



D D. Diaphragms. These are placed in 

 sectional and in face views. The diaphragm 

 is placed between the mirror and the illumi- 

 nator. In Fig. 20 the opening is eccentric for 

 oblique light, and in Fig. 22 the opening is a 

 narrow band, the central part being stopped 

 out, and thus giving rise to dark-ground il- 

 lumination. 



Obj. Ohj. The front of the objective. 



