Il6 DETERMINATION OF THE CHARACTER OF OBJECTS [Ch. IV 



than the liquid. For this method a series of liquids of known index 



of refraction must be at hand. For a complete discussion, see 



Chamot, Ch. IX. 



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, but under the liquid it 



can hardly be seen except where 



the red band is painted on it. 



This is a good example of a color 



Fig. 72. Solid Glass Rod Coated image and of a refraction image 

 with Collodion to Show Double ,, , , fi s 



Contour. to the naked e y e (§ x 37)- 



§ 202a. 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. 



§ 203. 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 

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

 and the medium in which it was mounted must differ considerably. 



§ 204. 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. 



This may be demonstrated by coating a fine glass rod {% 202) with 

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

 then mounting in 50% glycerin as above (§ 202). Employ a 4 mm. 

 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. 72). 



