98 INTERPRETATION OF APPEARANCES [CH. Ill 



§ 147. Optical 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 slightl}*. 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 studying mammalian 

 red blood-corpuscles on edge. The experiments with the solid glass 

 rods (Fig. 92) furnish excellent and striking examples of optical 

 sections. 



§ 148. Currents in Liquids. — Employ the 16 mm. (^ in.) ob- 

 jective, 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. 



§ 149. Velocity Under the Microscope. — In studying currents 

 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. 37 it 

 will be seen that the actual size of the field of the microscope 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 object moves across the field 

 it ma}* appear to move with great rapidity, whereas if one measures 

 the actual distance passed and notes the time, it will be seen that the 

 actual motion is quite slow. One should keep this in mind in stud)'- 

 ing the circulation of the blood. The truth of what has just been 

 said can be easily demonstrated in studying the circulation in the gills 



*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- 93)- 



