112 DETERMINATION OF THE CHARACTER OF OBJECTS [Ch. IV 



§ 194. Air bubbles. — Prepare these by placing a drop of thin 

 gum arabic mucilage on the center of a slide and beating it with a 

 scalpel blade until the mucilage looks milky from the inclusion of air 

 bubbles. Put on a cover-glass but do not press it down. 



§ 195. Air bubbles with central illumination. — Shade the object 

 and with the plane mirror light the field with central light (fig. 21 B). 



Search the prepara- 

 tion until an air bubble 

 is found appearing 

 about 1 mm. in diam- 

 eter, get it into the 

 center of the field, and 

 if the light is central, 

 the air bubble will ap- 

 pear with a wide, dark, 

 circular margin and a 

 small bright center. If 

 the bright spot is not 

 in the center, adjust 

 the mirror until it is. 



This is one of the 

 simplest and surest 

 methods of telling when 

 the light is central or 

 axial when no conden- 

 ser is used (§ 98). 



Focus both up and 

 down, noting that, in 

 focusing up, the central 

 spot becomes very clear and the black ring very sharp. On elevating 

 the tube of the microscope still more the center becomes dim, and the 

 whole bubble loses its sharpness of outline. 



§ 196. Air bubbles with oblique illumination. — Remove the 

 substage of the microscope and all the diaphragms. Swing the mirror 

 so that the rays may be sent very obliquely upon the object (fig. 67). 

 The bright spot will appear no longer in the center, but on the side 

 away from the mirror (fig. 68 A). 



Fig. 67. Oblique Illumination with a 

 Mirror and with a Condenser. 



A The light is shown to be oblique with ray c; 

 rays A B are central. The arrows indicate the 

 path of the rays. (For the objective see explan- 

 ation of fig. 35.) 



B Abbe condenser with an eccentric dia- 

 phragm (D) admitting light only on one side. 



Axis The principal optic axis. Ob Objective. 



S. Axis Secondary axis. 



