1S92.] 



MICliOSCOI'ICAL JOURNAL. 



83 



EXPERIMENTS. 



Abbe Illuminator, Axial and Oblique Light.— Use a dia- 

 pliragm a little larger than the front lens of the ^ (3 mm.) or No. 

 7 objective, have the illuminator on the level or nearly on the 

 level of the upper surface of the stage, and use the plane mirror. 

 Be sure that the diaphragm carrier is in the notch indicating that 

 it is central in position. Use the Pleurosignia as object. Study 

 carefully the appearance of the diatom vs^ith this central light. 



Explanation of Plate L 



Fig. 20, 20:', 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 the illuminator, and is therefore oblique to 

 the principal axis. 



A, Fig. 2 1 represents the upper part of the 

 illuminator. 



D D. Diathragms. 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- 



Ohj. Obj. The front of the objectiv 



then make the diaphragm eccentric so as to light with oblique 

 light. The ditlerences in appearance will probably be even more 

 striking than with the mirror alone. 



Lateral Swaying of the Image. — Frequently in studying an 

 object, especially with a high power, it will appear to sway from 

 side to side in focussing up or down. A glass stage micrometer 

 or fly's wing is an excellent object. Make the light central or 



