CH. Ill] INTERPRETATION OF APPEARANCES 103 



§ 147. Determination of the Form of Objects. — The pro- 

 cedure is exactly as for the determination of the form of large ob- 

 jects. That is, one must examine the various aspects. For ex- 

 ample, if one were placed in front of a wall of some kind he could 

 not tell whether it was a simple wall or whether it was one side of a 

 building unless in some way he could see more than the face of the 

 wall. In other words, in order to get a correct notion of any body, 

 one must examine more than one dimension, — two for plane sur- 

 faces, three for solids. So for microscopic objects, one must in some 

 way examine more than one face. To do this with small bodies in 

 a liquid the bodies may be made to roll over by pressing on one 

 edge of the cover-glass. And in rolling over the various aspects are 

 presented to the observer. With solid bodies, like the various 

 organs, correct notions of the form of the elements can be deter- 

 mined 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. 



§ 148. 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. (yi in.) or higher objective and a high ocular 

 for all the experiments. It may be necessary to shade the object 

 (§ 120) to get satisfactory results. When a diaphragm is used the 

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



§ 149. Air Bubbles. — Prepare these by placing a drop ot 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. 97. Diagram 



showing how. to place a _^J ( <o Jt-—' 1 — 'fff, j of e £ P* 



cover-glass upon an ob- 

 ject with the forceps. 



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

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

 . (Fig. 28). 



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