In case there is no opacity of the media of the eye, low central vision may be due to two causes : a refractive error or a centrally located defect in the sensorium. The separation of these two causes presents a somewhat difficult problem in diagnosis. Among the possibilities available the following may be noted: (a) An attempt may be made to improve vision by corrective glasses. (b) The eye may be refracted by one or more of the objective methods. (c) The influence of refractive errors may be reduced to a minimum by the use of a very small artificial pupil in close proximity to the cornea. (d) With the best corrective glass that can be obtained, an examination may be made for a central scotoma.
In the instrument to be described, the following selection is made from these possibilities: The eye is rendered practically independent of refraction, and