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ANNUAL EEPORT SMITHSONIAN INSTITUTION, 192 8 



mirrors, the resulting curve would be a paraboloid, and it is this 

 surface which our amateurs are seeking- when they attempt a perfect 

 mirror. 



Light of all colors is reflected by a mirror at the same angle, so 

 that the image appears in natural colors. In passing through a 

 single lens, however, the blue or shorter wave lengths are bent more 

 than the red rays and come to a shorter focus. This defect, called 

 chromatic aberration, makes the image formed by a simple lens not a 

 single clear picture but a sharp image of one color surrounded by a 

 halo of all the remaining colors. The reader can see this for hini- 



ABERRATION 

 SPHERICAL CHROMATIC 



\^ 



Figure 2. — Abrrrations of a Ions 



self by holding a reading glass between an incandescent lamp and 

 the wall and moving it slightly back and forth. 



A single lens with spherical curves has also another defect which 

 we call spherical aberration. If you cover first the center, then the 

 rim, of your lens, you will find a difference in focus for images even 

 of the same color. By choosing the proper curves this difference can 

 be greatly reduced. Both of these aberrations require a second lens 

 to bring the multiple images to a common focus. 



To-day we may choose from the glass manufacturers' lists two 

 glasses which have nearly the same relative dispersion or spreading 



