522 



APPENDIX 



(A) Chromatic Aberration. Put the ground-glass plate and the round 

 diaphragm in the slide-way of the lanterns. Make a pencil of parallel 

 rays and have them pass through the +10 D. lens placed 15 cm. away 



RED. 



VIOLET. 



v 



Refraction by media with oblique sides. A ray of white sunlight entering the prism ABC 

 obliquely is broken toward the perpendicular and away from it on leaving the denser medium. 

 The violet rays are refracted more than are the red rays of the spectrum. 



FIG. 289 



Chromatic aberration comes from lenses of certain materials because of the unequal refraction 

 of white light into its component rays. The violet end of the spectrum is bent more than the 

 red end, and the focus is indefinite, as in V R. This is corrected by using materials for lenses 

 which compensate this unequal refraction. 



from the ground-glass and just inside the eye (this distance is half-again 

 as long as the lens' focal distance). Place the card (white screen) 

 15 cm. farther beyond the lens. 



