426 THE OUTGO OF ENERGY 



finally horizontal the nearer focal line will be 

 vertical, the farther focal line will be horizontal. 



Kotats the cylinder through 45. 



By looking at the pencil first from one side of 

 the box and then from the other, the focal lines 

 may readily be seen in profile, as well as in 

 cross-section. 



ABERRATION 



Spherical Aberration by Reflection. In Fig. 

 64 a concave mirror, AB, has the centre of curva- 

 ture, C, and the principal focus, F. DE is one 

 of several incident parallel rays. CE is perpen- 

 dicular to the point of incidence. EFis the ray 

 reflected from E to the principal focus, G the 



point at which the reflected ray cuts the axial 

 line CK. DE\\CG, therefore /.GCE=CED 



