. The refraction of the eye; including a complete treatise on ophthalmometry; . Fig. 12. than the horizontal meridian, which latter fact makes it astig-matism with the rule. ASTIGMATISM AGAINST THE RULE 23 5. Mixed astigmatism with the rule. — The vertical meridianis myopic and focusses rays of light in front of the retina, andis more curved than the horizontal meridian which is hyper-metropic (fiat, less curved), and focusses rays back of the astigmatism is with the rule, therefore, though Fig. 13. We see, then, by the above five diagrams that astigmatismmay be with the rul
. The refraction of the eye; including a complete treatise on ophthalmometry; . Fig. 12. than the horizontal meridian, which latter fact makes it astig-matism with the rule. ASTIGMATISM AGAINST THE RULE 23 5. Mixed astigmatism with the rule. — The vertical meridianis myopic and focusses rays of light in front of the retina, andis more curved than the horizontal meridian which is hyper-metropic (fiat, less curved), and focusses rays back of the astigmatism is with the rule, therefore, though Fig. 13. We see, then, by the above five diagrams that astigmatismmay be with the rule in all of its forms; the only thing neces-sary to have it such, the sine qua non, so to speak, is that thevertical meridian of the cornea or one in its neighborhood shall hemore curved than the horizontal or the one at right angles to it. Where the vertical meridian of the cornea, or any meridianin the neighborhood of the vertical meridian, that is, within45° of the vertical, is less curved than the meridian at rightangles to it, that is astigmatism against the 7^ule. In other words, astigmatism against the rule means simplythat the vertical meridian of the cornea is less curved thanthe horizontal; and this condition may obtain in any formof astigmatism — hypermetropic (simple or compound), myopic(simple or compound), and in mixed astigmatism, as the fol-lowing diagrams show: — 4I 24 THE refractio:n^ of the eye 1. Simple hypermetropic astigmatism against the rule. —The vertical meridian is
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