104 BELL SYSTEM TECHNICAL JOURNAL 



Objectives are divided into four general classes: achromatic, senii- 

 apochromatic, apochroniatic and monochromatic for use with ultra- 

 violet light. These objecti\'es do not consist of single lenses but are 

 composed of two or more lenses very accurately centered and perma- 

 nently' mounted in a metal holder. The component parts of the lens 

 system are chosen with regard to their ability to correct or compensate 



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Fig. 4 — .A baltt-ry of low-power K-nsi-s. Those lenses are used without evepietes 

 Each lens is equipped with a diaphragm for stopping the aperture. 



for certain errors which are always characteristic of a simple lens. 

 The value of an objective depends on the degree to which these im- 

 perfections have been overcome. 



The difference in quality between the first three classes of ob- 

 jectives is primarily a matter of correction for chromatic and spherical 

 aberrations. Chromatic aberration is the inability of a lens to focus 

 sharply at the same point the different colors which go to make up the 

 incident light and the inability to bring two rays of incident light of 

 the same color to the same focus is termed spherical aberration. 



The achromatic objectives have the chief optical defects corrccttd 

 in a sutticient degree for the physiologically most effective rays (yellow- 

 green) of the visible spectrum, while in the case of the apochroniatic 

 objectives the correction of the image defects extends approximately 

 evenly over the entire range of the visible spectrum from the red 

 to the violet regions. 



In the achromatic lenses the fusion of the chromatic rays becomes 

 less and less complete for rays belonging to the extremes of the visii)le 

 spectrum under the ordinary conditions of illumination with white 

 light, and this imperfection becomes more apparent when higlil\ 

 magnifying e\epieces are used. There are also residual imperfec- 

 tions in the fusion of the ra\'s so that the colors of ol)jects are not 

 rendered with absolute precision in their finer shades. In the apo- 

 chroniatic objectives the fusion of the rays is so perfect that the>' may 

 be used in conjunction with high-power eyepieces, and because of 

 this perfect fusion the natural colors of the object are rendered with 



