Note on the use of lenses in series for sight testing

The paper deals with the theory of a train of lenses placed in series as used in a trial frame or other instrument. The series arrangement enables a small number of lenses to be combined to give the correction for any regular defect of form. It is shown that in general effective powers are not simply additive, but under conditions described in the paper the error made in regarding them as additive becomes small. The conditions involve the order of the lenses and the position of every lens except the first of the train. It is always possible to satisfy these conditions in a system designed for use in the way described. With incorrect arrangements serious errors may be made.