434 THE OUTGO OF ENERGY 



more axial rays, this focal point in the image will 

 be surrounded by dispersion circles made by the 

 rays which have been refracted from the periph- 

 ery through foci nearer the lens and which are 

 now diverging from these foci. If the screen be 

 placed at the principal focus for the peripheral 

 rays, this focal point will be surrounded by dis- 

 persion circles made by the rays that have not 

 yet converged to the principal axis. (Compare 

 spherical aberration, page 428.) 



Aberration avoided by a Diaphragm. Place 

 before the condenser the paper diaphragm with 

 1 cm. aperture. 



The image at once becomes distinct, and the 

 colors practically disappear. The outer rays, 

 which when refracted would cross the principal 

 axis far enough from the principal focus to cause 

 dispersion circles, have been cut off. When the 

 aperture of a lens or mirror is reduced by a 

 diaphragm to 10, the greater part of the spheri- 

 cal aberration is prevented. 



Spherical aberration is still further reduced by 

 combining several lenses in an objective (apla- 

 natic system). 



With an achromatic lens, consisting of a col- 

 lecting lens of crown glass united with a dispers- 

 ing lens of flint glass, all the spectral rays may 

 be brought to the same focus, and chromatic 

 aberration altogether avoided. 



