108 INTERPRETA TWN OF APPEARANCES [ CH. Ill 



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. 101). 



§ 160. Optional Section. — This is 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 will be in focus and a different 

 optical section obtained. The most satisfactory optical sections are 

 obtained with high objectives having large aperture. 



Nearly all the transparent objects studied may be viewed in 

 optical section. A striking example will be found in stud)'ing 

 mammalian red blood-corpuscles on edge. The experiments with 

 the solid glass rods (Fig. ioo)furnish excellent and striking examples 

 of optical sections. 



§ 161. Currents in Liquids. — Employ the 16 mm. (/lin.) 

 objective, and as object put a few particles of carmine on the middle 

 of a slide, and add a drop of water. Grind the carmine well with a 

 scalpel blade, and then cover it. If the microscope is inclined, a 

 current will be produced in the water, and the particles of carmine 

 will be carried along by it. Note that the particles seem to flow up 

 instead of down — why is this ? 



Lamp-black rubbed in water containing a little mucilage answers 

 well for this experiment. 



§ 162. Velocity Under the Microscope. — In studying cur- 

 rents or the movement of living things under the microscope, one 

 should not forget that the apparent velocity is as unlike the real 

 velocity as the apparent size is unlike the real size. If one consults 

 Fig. 42 it will be seen that the actual size of the field of the micro- 

 scope with the different objectives and oculars is inversely as the 

 magnification. That is, with great magnification only a small area 

 can be seen. The field appears to be large, however, and if any 



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

 phuric ether and 95% 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 membran- 

 ous covering (Fig. 101). 



