INTRODUCTION TO PHYSIOLOGICAL OPTICS 433 



their refrangibility ; violet crosses nearest the 

 lens, then blue, green, yellow, orange, and red in 

 the order named. The principal focus will thus 

 be a line of colors lying in the principal axis, the 

 end nearest the lens being violet. The peripheral 

 portions of a lens refract rays parallel to the prin- 

 cipal axis more strongly than the axial portion. 

 Hence the chromatic aberration will increase 

 with the aperture of the lens. 



Put the ground glass plate and the diaphragm 

 with 2 mm. aperture in front of the condenser. 

 Let the rays from the illuminated spot of ground 

 glass pass through the 10 D lens placed about 

 15 cm. in front of the ground glass, i. e. a dis- 

 tance somewhat greater than the focal distance 

 of the lens (10 cm.). Place a white screen about 

 15 cm. in front of the lens. 



The image of the white spot upon the ground 

 glass will be a disk with violet centre and red 

 margin. 



Eemove the white screen farther from the lens. 



At a distance of about 30 cm. the centre of the 

 image will be red and the border violet. 



The image in this experiment is blurred be- 

 cause the rays which pass through the peripheral 

 portion of the lens cross the principal axis sooner 

 than the rays which pass through the axial por- 

 tion. If the screen be placed at the focus of the 



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