The clinical study of ophthalmoscopic corneal images / by Ernest E. Maddox.

  • Maddox, Ernest E. (Ernest Edmund), 1860-1933.
Date:
[1892]
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Licence: Public Domain Mark

Credit: The clinical study of ophthalmoscopic corneal images / by Ernest E. Maddox. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    miss the natural discrepancy, and are apt to consider the case one of convergent strabismus. Differences in the angle y constitute so-called apparent strabismus, for the detection of which corneal images are invaluable. In hypermetropes the angle a (which we may consider for practical purposes as identical with the angle y) is generally greater than in emmetropes, the average being given by Donders as 7° 55'. 1 have noticed that in hypermetropic astig- matism, in which the horizontal meridian jwssesses the least curva- tnre, the angle is also generally greater than in emmetropia. In myopes the angle tends to be less than in emmetropes, and may be *' nil, in which case the two axes coincide ; or even negative, in which case tlie visual line lies on the other side of the optic axis. Donders gave the average in myopia as less than 2°. The angle y may be different in the two eyes. When this is the case, it is generally associated with a difference in refraction, but iiot always. It is of much clinical interest to know that the posi- tion of the corneal image when the eye is fixing the virtual source of light, or, in other words, the fixation position of the corneal image, maps out with fair precision that spot of the cornea which ]3 traversed by the visual line.^ It therefore generally lies, not in the centre of the pupil, but a little to the inner side of the centre, even though the pupil itself lies generally nearer the inner than the outer margin of the cornea. We are able, therefore, in a sense, to see what part of the cornea is traversed by the line of vision, and by tlie distance at which this point lies from the centre of the cornea we are able to guess approximately the amount of the angle^ ot the obliquity of vision in the eye under observation. Any instance of an unusually high or low angle at once strikes us, and may set us to try and account for it by looking for some abnormal condition of refraction, or unusual shape of the eye. It IS now a great number of years ago since corneal images were used to measure the radius of curvature of the cornea by the aid of the ophthalmometer invented by Hclmholtz. Eather later, the flame of a candle, movable along the arc of a perimeter, was 'used to measure strabismus, by noting the position of the candle on the arc requued to bring its image into the centre of the cornea of the 1 I arrive at this by taking the centre of curvature of the cornea as coin- cident witli the nodal point of the eye ; it is not really quite coincident, but the discrepancy is more or less neutralized by the ellipsoidal shape of the cornea and therefore for practical purposes the simplification is permissible 1 he angle a is the angle between tlie line of vision and the optic axis • the angle y is that between the line of fixation and the optic axis. The line ot vision ' extends from the object fixed through the nodal point to the lovea. Ihe line of fixation extends from the object fixed to the centre of rotation of the eye. The discrepancy between the angles a and y in a given eye is greater Cn the rearer the object fi.xed, and (2) tlie higher both angles are. ' w ^ Kii /'''^-^^ ^-'^ tiamuck //«?i(i6itc/i, Snellen and Landolt truly observe \Ve can for imictical pui'iioses (putc well take the angles y and a as equal to eacn oilier. —iJand in , p. -111. ^