(P. A.). That ray from the 

 object which is directed 

 straight towards the centre 

 of curvature or NODAL POINT 

 (N.P.) of the refracting sur- 

 face lies of necessity on one of 

 the radii to the surface; it is 

 therefore perpendicular to it 

 and is not refracted (ray 2). 

 If a pencil of rays are parallel 

 to one another and also 

 parallel to the principal axis 

 (see ray 1), they are all 

 brought to a focus after re- 

 fraction at some point on the 

 axis, this point being known 

 as the SECOND PRINCIPAL 

 FOCUS (P.F 2 , Fig. 40). This 

 lies nearer the surface if the 

 curvature on the surface is 

 sharp and the refractive index 

 high, farther back if the sur- 

 face is part of a large circle, 

 or if the refractive index of 

 the medium is not very great. 

 Light to be parallel must in 

 theory come from objects at 

 an infinite distance. In actual 

 fact, however, rays from 

 points more than about ten 

 metres away diverge so little 

 that, for the eye at least, 

 there is no practical differ- 

 ence between their focus and 

 that of rays which arise from 

 further away. All parallel 

 rays which reach the surface 

 inclined at a small angle to 

 the principal axis are brought 



