94 INTERPRETATION OF APPEARANCES [CH. Ill 



like the various organs, correct notions of the form of the elements can 

 be determined by studying sections cut at right angles to each other. 

 The methods of getting the elements to roll over, and of sectioning in 

 different planes are in constant use in Histology, and the microscopist 

 who neglects to see all sides of the tissue elements has a very inade- 

 quate and often a very erroneous conception of their true form. 



§ 135. Transparent Objects having Curved Outlines. — The 

 success of these experiments will depend entirely upon the care and skill 

 used in preparing the objects, in lighting, and in focusing. 



Employ a 3 mm. ( Vs in.) or higher objective and a high ocular for 

 all the experiments. It may be necessary to shade the object (§ 109) 

 to get satisfactory results. When a diaphragm is used the opening 

 should be small and it should be close to the object. 



§ 136. Air Bubbles. — Prepare these by placing a drop of thin 

 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. 



Fig. 90. Diagram show- 

 !j^£_j^liiK? ing how to place a cover- 



glass upon an object with the 

 forceps. 



§ 137. Air Bubbles with Central Illumination.— Shade the 

 object ; and with the plane mirror, light the field with central light 

 (Fig. 23). 



Search the preparation until an air bubble is found appearing 

 about 1 mm. in diameter, get it into the center of the field, and if the 

 light is central the air bubble will appear 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 condenser is used (§ 65). 



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. 



§ 13.S. Air Bubbles with Oblique Illumination.— Remove the 

 sub-stage of the microscope and all the diaphragms. Swing the mirror 

 so that the rays may be sent very obliquely upon the object (Fig. 23, 



