CH. Ill] INTERPRETATION OF APPEARANCES 107 



band simply represents an area in which the rays are so greatly bent 

 or refracted (Figs. 61-63) that they cannot enter the objective and 

 contribute to the formation of the image ; the edges are dark sim- 

 ply because no light from them reaches the observer. 



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. 



§ 158. Highly Refractive. — This expression is often used in 

 describing microscopic objects, (mednllated nerve fibers, for ex- 

 ample), and means that object will appear to be bordered by a wide, 

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

 above (§ 157), it would be known that the refractive power of the 

 object, and the medium in which it was mounted must differ con- 

 siderably. 



Fig. 10 r. Solid glass rod coated 

 with collodion to show a double con- 

 tour. Toward one end the collodion 

 had gathered in a fusiform drop. 



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

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

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

 line, (2) a light band, and (3) a second dark line (Fig. 101). 



This may be demonstrated by coating a fine glass rod (§ 157) 

 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 



* 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. 



