ON THE THEORY OF OPTICS. 419 



part of a flat image, of which the magnitude is determined by straight lines 

 drawn from the. extremities of the object through the centre of the 

 lens. This is, however, an approximation which is only admitted for 

 the greater convenience of computation and representation, the image 

 being almost always in reality considerably curved. (Plate XXVII. Fig. 

 386.) 



