Page:Handbook of Ophthalmology (3rd edition).djvu/48

42 emmetrope, with exactly the same amount of accommodative power, receives distinct retinal images from 4 inches up to an infinite distance.

The range of accommodation of myopic eyes may be determined by finding the positions of the far and near points, and reckoning from them the accommodation; or the examination may be made to resemble that of the emmetropic eye by removing the far point to an infinite distance, by the use of a neutralizing lens, and then finding the near point.

The behavior of the relative range of accommodation is of special interest. The myope learns unconsciously to converge nearly to his near point without accommodating. If, for instance, the far point lie at 8 or 10 inches, the eyes (of course without glasses) converge to nearly this distance without there being any simultaneous tension of accommodation.

Now, since myopes are much seldomer than emmetropes in a position to use their near points, it follows, by reason of the change in the relative range of accommodation, that less demand is made upon the accommodative power of the myopic than of the emmetropic eye.

In other respects, however, greater demands are often made on the myopic than on the emmetropic eye; particularly is this the case in reference to the convergence of the visual axes. If, with the naked eyes, the myope wishes to see distinctly and binocularly an object within his range of accommodation, his visual axes must converge to at least the distance of his far point. In only moderate degrees of myopia a stronger convergence is required than in emmetropic eyes. Moreover, the convergence is made difficult by two circumstances.

In emmetropic eyes the line of vision does not intersect the cornea at its centre, but somewhat toward the median side, A perpendicular let fall upon the centre of the cornea will form with the line of vision and on the external side of it an angle which we designate as the angle $$\alpha'$$. If the lines of vision are parallel, then the perpendiculars let fall upon the centre of the cornea must be divergent, and if the lines of vision intersect at any given point, the point of intersection of the perpendiculars must be at a greater distance. In myopia the deviation of the line of vision from the centre of the cornea is less than in emmetropia, and consequently