All the single lenses used in photographic combinations have either spherical or plane surfaces, and all, together with the diaphragm, are centred on an axial line which intersects every surface normally: which means that it is at right angles to all the plane surfaces and passes through the centres from which all the others are struck. This line is called the " principal axis " of the lens, and any light pencil joining object and image points which are situated on the principal axis is described as a "direct" pencil, to distinguish it from the oblique pencils which have foci off the principal axis. In testing the quality of a lens we consider mainly its performance in representing a plane object upon a parallel image plane, both planes being at right angles to the principal axis. Fig. 2 will serve to illustrate these test conditions, for the line p q is the principal axis of the lens l, while the object plane o, and the image plane b, are both at right angles to p q. The pencil from p to q is a " direct" one, while that from p to q is, of course, oblique. For experimental purposes pin-holes in o, with a bright light behind each, form good object points, and with this arrangement the aberrations about to be described can be easily identified.