the vertical meridian, then only the horizontal lines cast distinct retinal images. For the same reasons would one or the other of the inclined lines appear distinct if the direction of the principal meridians were not exactly vertical or horizontal. It is next to be experimentally ascertained what concave or convex cylindrical lens corrects the meridional asymmetry and causes all the lines to appear equally distinct.
In order to determine more exactly the condition of refraction in each of the principal meridians, a stenopaic slit with a breadth of 1 or 2 millimetres should be held before the eye in a direction corresponding with that of the line which is seen most distinctly; the condition of refraction in the principal meridian is then to be determined in the usual manner by the aid of concave or convex lenses; the slit is then to be placed at an angle of 90° with its former position, and the condition of refraction in the second principal meridian to be determined in the same way. The difference between the condition of refraction in the two principal meridians shows the degree of the astigmatism.
If one of the principal meridians is emmetropic, Donders calls the asymmetry "simple astigmatism." He distinguishes a simple myopic and a simple hypermetropic form; generally, in the myopic form the emmetropic principal meridian is horizontal, and in the hypermetropic form it is vertical.
Compound astigmatism is where the same anomaly of refraction exists in both principal meridians, but in a different degree; there may be myopic or hypermetropic compound astigmatism.
Mixed astigmatism is where myopia exists in one principal meridian and hypermetropia in the other.
Donders[1] has made numerous ophthalmometric measurements of the curvature of the cornea in cases of regular astigmatism. He has proved that a considerable meridional asymmetry of these curvatures must be regarded as the principal cause of this anomaly of refraction. Still, he found that neither the degree of astigmatism, as calculated from the difference of curvature of the cornea in the principal meridians, nor the direction of those meridians, corresponds absolutely with the actual condition of refraction of the entire refracting apparatus of the eye, obtained experimentally.
- ↑ Arch. f. Ophth., B. x. 2, pag. 83.
