The first‐order focusing properties of a magnetic spectrograph similar to that of Siegbahn and Svartholm, but differing in that both source and collector lie outside the field, are investigated. The parameters of the analogous thick lens optical system are derived. General expressions for image distance, astigmatism, magnification, solid angle, dispersion, and resolution of lines with a spread in momentum are obtained. Factors affecting the compromise between intensity and resolution are discussed. The possibility of increasing resolution without loss of intensity by introducing some astigmatism is considered. Second‐order effects have not been included since they are thought to be of the same order as corrections for the fringing field. The first approximation proved adequate for the design of an instrument in which satisfactory focusing was obtained by simple empirical adjustments.