THE MICROSCOPE 37 



slide. Air has a refractive index of 1.00, while the 

 refractive index of glass is approximately 1.55. By 

 placing a layer of water, which has a refractive index 

 of 1.30, between the objective and the slide, we mini- 

 mize these differences in refractive index, thus decreas- 

 ing the number of light rays lost through refraction. 

 Thickened cedar oil, of refractive index 1.55, is usually 

 used as an immersion medium in bacteriological work 



9 



_^ 



2 



3 



4 



FIG. 20. Immersion Objectives. 



A. Unless immersion fluids are used, considerable light is lost through refrac- 

 tion. B. Very little light is lost through refraction if immersion fluids are used. 

 1. End lens of objective. 2. Space between objective and coverslip. 3. Cover- 

 slip. 4. Space occupied by specimen. 5. Object slide. 



(Fig. 20). Lenses which cannot be used without the 

 interposition of liquids to prevent excessive loss of 

 light are termed immersion objectives, and the liquids 

 used for this purpose are known as immersion liquids. 

 Immersion lenses, while of importance in bacterio- 

 logical and pathological investigations, are seldom 

 used in vegetable histology. As immersion lenses 

 are very expensive and easily scratched they must 

 be handled with care, and immersion fluids must be 



