CH.III] INTERPRETATION OF APPEARANCES 97 



Fig. 92. Solid glass rod showing the 

 appearance when viewed with transmit- 

 ted, central light, and with an objective 

 of medium aperture. 



a. Mounted in air. b. Mounted in 50 per cent, glycerin. 



If the glass rod or any other object were mounted in a medium of 

 the same color and refractive power, it could not be distinguished from 

 the medium.* 



A very striking and satisfactory demonstration may be made by 

 painting a zone or band of eosin or other transparent color on a solid 

 glass rod, and immersing the rod in a test tube or vial of cedar oil, 

 clove oil or turpentine. Above the liquid the glass rod is very evident, 

 as it is also at the colored zone, but at other levels it can hardly be 

 seen in the liquid. 



§ 145. Highly Refractive. — This expression is often used in de- 

 scribing microscopic objects, (medullated nerve fibers, for example), 

 and means that the object will appear to be bordered by a wide, dark 

 margin when it is viewed by transmitted light. And from the above 

 (§ 144), it would be known that the refractive power of the object, and 

 the medium in which it was mounted must differ considerably. 



§ 146. Doubly Contoured. — This means that the object is 

 bounded by two, usually parallel dark lines with a lighter band between 

 them. In other words, the object is bordered by (1) a dark line, (2) a 

 light band, and (3) a second dark line (Fig. 93). 



This ma}' be demonstrated by coating a fine glass rod (§ 144) with 

 one or more coats of collodion or celloidin and allowing it to dry, and 

 then mounting in 50% glycerin as above. Employ a 3 mm.(^ in.) or 

 higher objective, light with transmitted light, and it will be seen that 

 where the glycerin touches the collodion coating there is a dark line — 

 next this is a light band, and finally there is a second dark line where 

 the collodion is in contact with the 

 glass rod.* (Fig. 93). 



Fig. 93. Solid glass rod coated with col-\ 

 lodion to show a double contour. Toward ' 

 one end the collodion had gathered in a fusi- 

 form drop. 



*Some of the rods have air bubbles in them, and then there results a capillary 

 tube when they are drawn out. It is well to draw out a glass tube into a fine 

 thread and examine it as described. The central cavity makes the experiment 

 much more complex. 



