Ch. IX] 



CORRECTION OF ABERRATIONS 



289 



White 



Axis 



Fortunately the visible spectrum does not include a greater range 

 of wave lengths (tig. 151), and if it were markedly less the optician 

 would find his task greatly lightened. As shown in fig. 139, the bright- 

 est region of the spectrum to the eye is really limited, and the old 

 opticians made good instruments for visual purposes by overcoming 

 the aberrations in large part in this very limited region ; but with the 

 requirements of photography and 

 for the most complete visual study 

 of the phenomena and objects of 

 nature by means of optical in- 

 struments greater and still greater 

 demands were made for optical 

 instruments including at least the 

 whole visible spectrum, and for some 

 purposes extending into the infra- 

 red and the ultra-violet. 



§ 464. Correction of the aberra- 

 tions of lenses. — From the very 

 law of refraction bound up with 

 the different wave lengths of visible 

 light it would seem impossible to 

 obtain the refraction necessary to 

 produce images (fig. 163-166) with- 

 out at the same time dividing the 

 light up into its colors. If the re- 

 fraction of each wave length were 

 in exact proportion to its length, as 

 with a diffraction grating it would be impossible to produce achro- 

 matic images. Newton thought the refraction was always as with a 

 grating, and he explained the satisfactory images produced by lenses 

 on the ground that the narrow part of the spectrum most brilliant to 

 the eye overwhelmed the dimmer parts so that the colored images on 

 both sides of the visual image were ignored. 



If one compares, however, the spectrum produced by the diffraction 

 grating (fig. 146) with that produced by a glass prism (fig. 147) it 

 will be seen that the refraction of the different wave lengths (disper- 



Light 



Fig. 171. Chromatic Aberration 

 with Composite Light. 



White light A beam of white light 

 composed of all the colors meeting a 

 lens and the different wave lengths 

 being differently refracted breaks the 

 composite light up into its constitu- 

 ent colors. 



Red Blue The long waved red 

 light is less refracted than the shorter 

 waved blue light. After crossing at 

 the foci the blue light is on the outside 

 of the diverging cone. 



fb, fr The focus of the blue light 

 (fb) nearer the lens than the focus of 

 the red light (fr). 



Axis The optic axis of the lens. 



The dispersion or separation into 

 colors differs with different transpar- 

 ent substances, and is not in propor- 

 tion to the mean refraction. 



