The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . of it, is,as a rule (perhaps in 75 to 80 per cent of all cases of astigma-tism), more curved than the horizontal, or the meridian at rightangles to it, the astigmatism in such cases is said to be accord-ing to the rule, or with the rule. French writers often callthis direct astigmatism, while they designate astigmatismagainst the rule as indirect astigmatism. Many times beginners ask if hypermetropic astigmatism isnot always with the rule, and if myopic astigmatism i
The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . of it, is,as a rule (perhaps in 75 to 80 per cent of all cases of astigma-tism), more curved than the horizontal, or the meridian at rightangles to it, the astigmatism in such cases is said to be accord-ing to the rule, or with the rule. French writers often callthis direct astigmatism, while they designate astigmatismagainst the rule as indirect astigmatism. Many times beginners ask if hypermetropic astigmatism isnot always with the rule, and if myopic astigmatism is notalways against the rule. As a matter of fact, it makes no dif-ference whether the astigmatism be hypermetropic (simple orcompound), myopic (simple or compound), or even mixed, justso the vertical meridian of the cornea or one within 45° of it ismore curved than the meridian at right angles to it, that isastigmatism with the rule. In order that the reader cannot possibly go astray on thispoint, I will take an example of each form of astigmatism, andshow by diagrams how it may be with the rule in every VERT. MER. Fig. 9. 1 Under the heading of astigmatism against the rule, I show how astigmatismmay be against the rule in all its forms. ASTIGMATISM WITH TIIK RULE 21 1. Simple hypermetropic astigmatism with the rule.— FigureNo. 9 shows the vertical and horizontal sections of an eye withsiu-li an error of refraction. It is seen by this diagram that thevertical meridian is emmetropic and allows the rays of light to torus on the retina, and is more curved than the horizontalmeridian which is flat and allows the rays of light to focus backof the retina. Therefore, according to our definition of astig-matism with the rule, this must be a case of it, for the verticalmeridian is more curved than the horizontal. •1. Compound hypermetropic astigmatism with the rule. — Insuch a case the vertical meridian is flat and allows the rays of
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