SCIOPTICON MANUAL. 



19 



CHROMATIC ABERRATION, OR DISPERSION OF COLOR. 



White light is separated by a prism into the seven 

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

 red. 



As a lens is analogous to a system of prisms, and as 

 violet is more refrangible than red, the violet rays v v 

 (Fig. 9) will intersect the axis closer to the lens than 

 the red rays r r. This error is corrected by combining 

 a concave lens of flint-glass with a convex lens of crown- 

 glass, so as to neutralize their contrary dispersions. 



The concave flint-glass lens / (Fig. 12), which has 

 great dispersive power in proportion to its curves, 

 diverges the violet more than the red, while the convex 

 crown-glass lens converges the violet more than the 

 red, so we have in both combined an achromatic convex 

 lens. As the chemical rays are in the violet end of the 

 spectrum, the photographer may succeed with an im- 



Flg. 9. 



perfectly corrected lens by having the sensitive plate a 

 little nearer the lens than the focus of luminous rays on 

 the ground-glass would indicate. Lenses without chro- 

 matic aberration are called achromatic. The term 

 aplanatic means without wandering, and may apply to 

 lenses corrected of both spherical and chromatic aber- 

 ration. 



