The mere statement of lens powers and the word "bifocals" do not constitute a complete or safe prescription. Most refractionists are confident that the finished glasses will duplicate the test lenses accepted by the patient during examination. If the patient sees well with the test lenses, he will, they assume, see comfortably and equally well with glasses purchased on the analogous prescription. The type of bifocal lens used is of little importance or is not their specific concern. A number of special factors which have a bearing on the selection of the most suitable type of bifocal lens are known only to the refractionist. These include a knowledge of the whole of the patient's refractive condition ; the visual acuity expected in each eye ; the presence of amblyopia in one eye; the muscle balance; the distance for which the reading portion was focused ; the seeing field which is