A general qualitative analysis of the aberrations of a thin lens system is given and applied to a triplet lens form which is quite flexible in adjustment of those aberrations which are capable of being varied in thin lens systems, particularly with regard to the axial secondaries, zonal spherical aberration, and sphero-chromatism. In its highly corrected state the lens form, of which three general examples are given, may be made to fulfill the conditions of Fraunhofer and Gauss at high relative apertures with mild curves and insensitive spacing