The nodal slide is only convenient for the examination of lenses over their entire field when these are of normal type.Collimators and lenses should in general be directed to pass through all useful light as symmetrically about their axes as possible. Failure to secure this condition may lead to an underestimate of the excellence of good lenses as well as an underestimate of the gravity of the faults present in bad lenses.The nodal slide may with advantage be supplemented by suitable linkages, which enable all the special conveniences of the nodal slide to be retained. Some of the possible modifications are discussed.