33 INTERPRE TA TION OF APPEARANCES. 



the fragment of cover-glass in that. The dark contour will be much 

 narrower than before. 



Draw a solid glass rod out to a fine thread. Mount one piece in air, 

 and the other in 50% glycerin. Put a cover-glass on each. Employ 

 the same optical arrangement as before. Examine the one in air first. 

 There will be seen a narrow, bright band, with a wide dark band on 

 each side. 



The one in glycerin will show a much wider bright central band, 

 with the dark borders correspondingly narrow (PI. Ill, Fig. 24). 



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.* 



§ 90. Highly Refractive. — This expression is often used in describ- 

 ing microscopic objects, (medulated nerve fibres 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 

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

 the medium in which it was mounted must differ considerably. 



§ 91. 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 (PI. Ill, Fig. 25). 



This may be demonstrated by coating a fine glass rod (§ 89) with 

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

 then mounting in 50% glycerin as above (§ 89). Employ a 5 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 rodf (PI. HI, 

 Fig. 25). 



§ 92. Optical Section. — The appearance obtained in examining 

 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 



* 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. 



t The collodion used is a 5 per cent, solution of gun cotton in equal parts of 

 sulphuric ether and 95 per cent, alcohol. It is well to dip the rod two or three 

 times in the collodion 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 mem- 

 branous covering (Fig. 25). 



