Ch. II] EXPERIMENTS WITH A SUBSTAGE CONDENSER 



65 



of the condenser be in part nullified (see Ch. IX for critical angle). 

 By making the homogeneous connection between the slide and con- 

 denser, any light leaving the condenser passes on through the slide 

 without change of direction, except any that may be given by the 

 object. 



The centering of the homogeneous immersion condenser is of 

 prime importance, and practically all forms of them are supplied with 

 centering screws for the pur- 

 pose (tig. 40, § 104). 



§ 114. Use of artificial 

 daylight. — Place a piece of 

 the daylight glass in the 

 opening of the dark screen 

 or lantern (fig. 37-38). If 

 this daylight glass is lightly 

 ground, it will diffuse the 

 light without lowering its in- 

 tensity too greatly. 



If possible, use a 100 

 watt nitrogen-filled tungsten 

 (mazda) concentrated filament lamp. Put the lamp within 5 to 

 10 cm. of the daylight glass. Use low and high powers and the 

 various colored specimens that require daylight to bring out 

 their color values. No matter how delicate the coloring or what 

 the tint is, it will be as faithfully presented by the artificial day- 

 light as by natural daylight. One can prove this by having the 

 apparatus near a window, then the mirror can be turned toward 

 the artificial or the natural daylight at will and the color effects can 

 be compared. 



By using the daylight glass with other artificial lights it will be seen 

 that even those lights when filtered through this glass give remarkably 

 good daylight effects. 



§ 115. Axial and oblique light with the condenser. — To demon- 

 strate the effect of the methods of illumination when a condenser is 

 used, take any striking preparation like a diatom (Pleurosigma angu- 

 latum, for example); employ a 4 mm. objective. Being sure that 



Fig. 43-44. Centering the Condenser 

 and the Source of Light. 



A The spot of light in the back com- 

 bination of the objective at one side, show- 

 ing that the condenser is not centered. 



B The spot of light in the center, show- 

 ing that the condenser is centered. 



C The source of light is not in the 

 middle. 



D The source of light is centered and il- 

 luminates the object. 



