126 BACTERIOLOGY. 



credit for introducing', in 1825, the method of correcting' 

 chromatic aberration. The real value of a microscope, as 

 an aid in scientific research, dates from this period. These 

 improved objectives, since the color defects were done away 

 with, were designated as achromatic objectives. This 

 achromatism was obtained by the combination of two 

 glasses, crown and flint. 



The dispersive power of flint glass is almost double tha:t 

 of crown glass, and hence, a crown bi-convex lens is fitted 

 into a plano-concave leiis of flint glass. The effect of this 

 combination will be understood, if a prism of crown glass, 

 having an angle of 50°, be placed by the side of an inverted 

 one of flint glass, having an angle of 25°. The ray of light 

 passing through the crown glass is decomposed into its 

 components, the colored rays of the spectrum. These then 

 enter the inverted flint glass prism, and are reconstituted 

 to white light. The concave flint lens is virtually an in- 

 verted prism, with reference to the convex, crown glass lens. 

 Refraction is thus obtained without dispersion/ 



Even the best achromatic objective gives an image 

 that is not wholly free from color. This is due to the fact 

 that the two kinds of glass employed do not disperse the 

 several rays in the same proportion. For instance, the 

 green ray may occupy the exact middle of the spectrum 

 produced by one glass, whereas, in that produced by the 

 other, it may lie to one side, possibly only one-third of the 

 distance from the red to the violet. If, therefore, the red 

 and violet rays are neutralized, the green will still remain. 

 Consequently, there is always a certain amount of color 

 present around the image of an achromatic objective. This 

 is known as the secondary spectrum. 



About .10 years ago Abbe and Zeiss prepared special 

 kinds of glass, the so-called borate and phosphate glass, 

 by means of which it was possible to do away with the 

 secondary spectrum. These objectives, which represent 



