38 Rubens and Trowbridge — Dispersion and Absorption 



diffraction images in an opposite direction. Hence, the left 

 image is more spread out, while the right one, from the same 

 cause, becomes narrower and sharper, since in the first case 

 the dispersions of both prism and grating are added, while in 

 the second case, only the difference of the two dispersions has 

 an effect, as in an achromatic lens. 



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