CH. III.] INTERPRETATION OF APPEARANCES. 87 



A very striking and satisfactory demonstration may be made by paint- 

 ing 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. 



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

 scribing microscopic objects, (medulated 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 

 (§ 1 -'h'i), it would be known that the refractive power of the object, and 

 the medium in which it was mounted must differ considerably. 



§ 137. 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 

 baud, and (3) a second dark line (Fig. 90). 



This may be demonstrated by coating a fine glass rod (§ 135) 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. (}i 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. 90). 



Fig. 90. Solid glass rod coated with col- 

 lodion to show a double contour. Toward 

 one end the collodion had gathered in a fusi- 

 form drop. 



§ 138. Optical Section. — This is the appearance obtained in examin- 

 ing transparent or nearly transparent objects with a microscope when 

 some plane below the upper surface of the object is in focus. The upper 

 part of the object which is out of focus obscures the image but slightly. 

 By changing the position of the objective or object, a different plane will 

 be in focus and a different optical section obtained. The most satisfac- 

 tory optical sections are obtained with high objectives having large 

 aperture. 



Nearly all the transparent objects studied may be viewed in optical 



* The collodion used is a 6% solution of gun cotton in equal parts of sulphuric 

 ether and 95% alcohol. It is well to dip the rod two or three times in the collo- 

 dion and to hold it vertically while drying. The collodion will gather in drops, 

 and one will see the difference between a thick and a thin membranous covering. 

 (Fig. 90). 



