ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES 49 



fill the annular entrance space. The radiant and a suitable 

 condensing lens are then so placed as to yield parallel rays and 

 produce a spot of light of the proper size and intensity at the 

 center of the plane mirror of the microscope, the mirror being 

 so inclined as to reflect the light rays into the dark-ground illu- 

 minator. Dark-ground illuminators require that an immersion 

 fluid be placed between them and the object slide. Usually 

 freshly filtered water is sufficient, although homogeneous im- 

 mersion oil sometimes yields better results, especially with illu- 

 minators of the plate type. 



In applying the immersion fluid and laying the object slide 

 in place great care must be taken to prevent the entrance of 

 air bubbles or dust particles. 



Because the light rays are caused to emerge from the illumina- 

 tor at such an angle (determined by the inclination of the reflect- 

 ing surfaces) as to converge to an axial point lying just above 

 the plane of the object upon the object slide, it is, of course, 

 essential that the thickness of the object slide be known, for if 

 too thin the illuminating rays will meet too far above the material 

 to be studied, or if too thick the focal point will lie too low; 

 for these reasons optical instrument makers mark upon the de- 

 vices the object slide thickness to be employed. For example: 



Thickness of object slide. 



Bausch and Lomb paraboloid illuminator 



Zeiss paraboloid condenser 



Reichert reflecting condenser 



Reichert slip-in reflecting condenser 



Leitz reflecting condenser 



Zeiss cardioid condenser for quartz cell 



i .40 to i .55 mm. 

 i .o to i . 10 mm. 

 0.7 to 1. 10 mm. 



2.0 mm. 

 less than i mm. 



i .2 mm. 



Absolutely clean object slides and cover glasses are essential 

 and great care must be exercised in wiping off the immersion 

 fluid from the condenser to avoid scratching the glass. Lens 

 paper of the highest grade only should be employed, and the 

 wiping off of the fluid should be done with the least pressure 

 possible, otherwise fatty material from the fingers may be forced 

 through the pores of the lens paper upon the glass. A mere 



