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most common form is the double convex and when 

 considerable magnifying power is desired, the 

 defects of spherical aberration are partially over- 

 come by the interposition of an opaque plate with 

 round opening of such size that it will shut out the 

 marginal rays. This is called a diapliragni and 

 when used the lens is said to be stopped down. 



Chromatic Aberration. -- In magnifying an 

 object with a single lens it will be noticed that it 

 has not only the defect of spherical aberration, but 

 that the object is fringed with colors, predomi- 

 nantly violet and red, or if objects are viewed 

 through a pri<m, we have not only an apparent 

 change of position, but a decided appearance of 

 so-called rainbow colors. This appearance is 

 called chromatic aberration and is a result of 

 refraction. It is caused by the dispersion, or 

 dispersive quality, the separation of light into its 

 primary colors, violet, indigo, blue, green, yellow, 

 orange and red in the order given. As the light is 

 refracted by a lens or prism on its emergence, the 

 violet end being more refrangible will be princi- 

 pally affected and be brought to a focus within, 

 and the red to a focus beyond the principal focus. 

 This band of colors is called the spectrum and is 

 nicely illustrated in the diamond. This has a very 

 high degree of refractiveness and is polished or 

 cut to make a many-sided prism in which each 

 face or facet creates refraction with its consequent 



