150 



Animal Micrology 



Flatness of Field. See aplanatism. 



Homogeneous Immersion Objective. See immersion objective. 



Illumination. Any means employed to direct light upon the object 

 under observation. Light which traverses the object is said to be trans- 

 mitted light. Most microscopical work in biology is done by means of 

 transmitted light, hence the object must be rendered more or less trans- 

 parent if not naturally so. If the object is symmetrically lighted, the 



FIG. 64. Dissecting Microscope. 



lighting is designated as axial or central illumination. If one side is 

 lighted more than another, the term oblique illumination is employed. 

 In the case of transmitted light, the light which traverses the object is 

 usually light reflected from a mirror because it is generally inconvenient 

 or impossible to hold the instrument directly toward the source of light. 

 Light which falls upon the object and is reflected from it to the eye, 

 either directly or through a microscope, is termed reflected light. Such 

 illumination is employed but little in ordinary histological work, but it 

 is useful in the examination of opaque objects such as metals, insects, 



