Ch. IV] DETERMINATION OF THE CHARACTER OF OBJECTS 117 



§ 204a. The collodion used is a 6% solution of soluble cotton in equal 

 parts of sulphuric ether and 95%, or, preferably, absolute alcohol. It is well 

 to dip the rod two or three times in the coUodion and to hold it vertically while 

 drying. The coUodion will gather in drops, and one will see the difiEerence 

 between a thick and a thin membranous covering (fig. 72). 



§ 205. Optical section. — This is the appearance obtained in ex- 

 amining 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 objec- 

 tives having large aperture. 



Nearly all the transparent objects studied may be viewed in optical 

 section. A striking example will be found in stud3dng Mammalian 

 red blood corpuscles on edge. The experunents with the soHd glass 

 rods (fig. 71) furnish excellent and striking examples of optical sec- 

 tions. 



§ 206. Currents in liquids. — Employ a 16 mm. objective, and as 

 object put a few particles of carmine, starch, or chalk dust on the 

 middle of a shde and add a drop of water. Grind the carmine or 

 other substance well with a scalpel blade; leave the preparation 

 uncovered. If the microscope is inchned, a current will be produced 

 in the water, and the particles will be carried along by it. Note that 

 the particles seem to flow up instead of down; why is this? How 

 would it appear to flow with an erecting microscope (§ 146, 149a)? 



§ 207. Velocity under the microscope. — In studying currents or 

 the movement of Uving 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. 29 it will 

 be seen that the actual size of the field of the microscope with the differ- 

 ent 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 may 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 studying 



