140 



Animal Micrology 



rays) meet at a point farther away (Fig. 53). This failure of the 

 color rays to meet in one focal point is termed chromatic aberra- 

 tion, and if uncorrected causes the image of an object viewed 

 through such a lens to be bordered 

 by a colored halo. 



The defect is corrected by prop- 

 erly combining glasses of different 

 dispersive powers but of kindred 

 refractive powers. Flint glass 

 (silicate of potassium and lead), 

 for example, has a dispersive power equal to about twice that of 



crown glass (silicate of potas- 

 sium and lime), although their 

 > J lhe0cutar refractive powers are nearly the 

 same. By combining a bicon- 

 vex lens of crown glass with a 

 concave lens of flint glass so 

 constructed that its dispersive 

 power will just equal that of the 

 crown glass (Fig. 54), the error 

 may in large measure be cor- 

 rected. Such an arrangement 

 does not interfere seriously with 

 the refractive powers of the lens 

 so constructed. Unfortunately 

 no two kinds of glass have been 

 found which have proportional 

 dispersive powers for all colors, 

 so that in the ordinary achro- 

 matic objective only 

 two of the different 

 colors of the spec- 

 trum have been ac- 

 curately corrected 

 and brought to one 

 focus. The colors 



FIG. 54. 



...Draw TubeDiaphragm 

 wilh Society Screw 



of the 

 Obje.cU/e 



-Slide' 



FIG. 52. Sectional View of Microscope Tube 

 including Ocular and Objective. 



