152 



Animal Micrology 



The objectionable yellowness of most artificial light may be elim- 

 inated by interposing a piece of green signal glass between the lamp and 

 the microscope. With most microscopes, round slips of blue glass which 

 fit into the substage mechanism are supplied for this purpose. Many work- 

 ers still employ as a screen an ammonia sulphate of copper solution in a 

 globular flask. To make the solution dissolve a small amount of copper 

 sulphate in water, and add ammonia. At first a precipitate appears, but 

 if an excess of ammonia is added this is dissolved and a transparent 

 deep-blue liquid results. This should be diluted with water sufficiently 

 to get a blue of just the proper depth to render the transmitted light 

 white as seen through the microscope. The globular flask also acts as a 

 condenser. 



Immersion Objective. A kind of objective in which a liquid is used 

 between the front lens and the cover-glass. Cedar oil is the most widely 

 used medium. In as much as the optical properties of cedar oil (refrac- 

 tion and dispersion) are almost the same as crown glass it is often termed 

 a homogeneous immersion fluid. A homogeneous immersion lens, 

 therefore, would be one intended for use with such a fluid. The advan- 

 tage of an immersion over a dry lens lies in the fact that, other things 

 being equal, after leaving the cover-glass rays which would be so 

 refracted in a rarer medium like air as to miss the front end of the objec- 

 tive, reach this lens in the case of immersions and traverse the objective. 

 With homogeneous immersions the rays of light are carried without 

 deflection through cover-glass and fluid and into the glass of the front 



lens. Water has a greater density than air 

 and less than glass, hence, with a water 

 immersion more rays of light reach the 

 front lens than with a dry lens, and less 

 than with a homogeneous immersion lens 

 (Fig. 67). The effect of an immersion is 

 practically to widen the angle of the lens 

 (see angular aperture}. 



Magnifying Power. The power of a lens 

 to multiply the apparent dimensions of an 

 object viewed through it. It should be 

 expressed in diameters not in areas. While 

 magnifying power is very important it is 

 only so in connection with resolving power. 

 If high power were the only essential, a 

 series of single lenses might be used. The 

 impossibility of using such a series for high 

 FIG. 67. (From Bausch, "Manipu- magnification is due to the fact that proper 



lation of the Microscope.") 



OIL. 

 111. 



