Handbook of ophthalmology . lmoscopic examination. For the functional examination itis best to use certain systems of lines. Javals* optometer Is acontrivance with which to determine the existence of of this complicated Instrument we may use a system oflines placed at a definite distance; the patient looks at these lines * Annales dOculistique; 1866. 68 ASTIGMATISM. with each eye separately, and it is determined experimentallywith what lens his vision is most improved. Among these sys-tems of lines are Beckers, and the letters of Pray and Heymann,or, perhaps the very best,
Handbook of ophthalmology . lmoscopic examination. For the functional examination itis best to use certain systems of lines. Javals* optometer Is acontrivance with which to determine the existence of of this complicated Instrument we may use a system oflines placed at a definite distance; the patient looks at these lines * Annales dOculistique; 1866. 68 ASTIGMATISM. with each eye separately, and it is determined experimentallywith what lens his vision is most improved. Among these sys-tems of lines are Beckers, and the letters of Pray and Heymann,or, perhaps the very best, those which accompany Snellens testletters, and which are shown on a reduced scale in Fig, 13, Thelines are arranged in the form of a half-star, since a figure of thatshape embraces lines running in every conceivable deviation from the perpendicular is expressed in degrees,which are marked with the positive sign on the right side of theperpendicular and with the negative sign on the left. Fig. 13. oVcrtLcaL^. The lines are placed at a distance suited to the vision of thepatient, and he is directed to look at them with one eye while theother is covered. If concave or convex glasses improve distantvision, the examination is to be conducted with their help. Careshould be taken that the head be held erect, and that the eyes bewell opened, so that the palpebral fissure does not act as a steno-paic slit. If, for instance, there be emmetropia in the horizontaland myopia in the vertical meridian, then will a distant luminouspoint throw its image upon the retina in the form of a verticalline, since the posterior focal line, which in this case is vertical,falls with relaxed accommodation exactly on the retina. Consequently, only the vertical lines in the figure will be seendistinctly, the others being less sharply defined. If, on the con-trary, there be hypermetropia in the horizontal and emmetropia in ASTIGMATISM. 69 the vertical meridian, then only the horizontal lines
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