English: Diagram showing the conditions which obtain where an extended radiant field is employed as the source of light. (With a view to facilitating the elucidation of these conditions, a parti-coloured red and blue screen serves in this and the succeeding diagrams in each case as the source of light.)

A. Diagrammatic representation of the image which is formed in the diaphragm of the ocular. The red disks here represent the antipoints corresponding to the radiant points in the central portion of the object field which send their light in the form of wide unmutilated beams symmetrically through the aperture of the objective; the blue elliptical figures on the periphery which extend inwards over the image field represent the elliptical antipoints corresponding to the radiant points in the periphery of the object field which send through the aperture of the objective beams which are cut down in an unsymmetrical manner by the edges of the objective mount.

B. Diagrammatic representation of the apertural plane of the objective where the blue and red beams intersect to form nodal points which radiate as shown in Plate VII., Fig. 3 (A.P.), a mixture of blue and red rays. To be noted here is also the fact that the apertural plane of the objective is fully filled in by the transmitted beams (vide p. 219).