The student's guide to diseases of the eye . specially when the lens ismoved from side to side. It is seldom much bene-fited by glasses. Returning to Regular Astigmatism, it will be seenthat the optical condition of the eye depends uponthe position of the retina in respect to the focalinterval. In the following diagram (Fig. 109) let themost refract-ing meridian bevertical, and itsfocus be calleda, the least re-fracting meri-dian its focus b. (The astigrna- *ig. 109. tism is here re-presented as caused by altered position of the retinain different planes, instead of by altered c
The student's guide to diseases of the eye . specially when the lens ismoved from side to side. It is seldom much bene-fited by glasses. Returning to Regular Astigmatism, it will be seenthat the optical condition of the eye depends uponthe position of the retina in respect to the focalinterval. In the following diagram (Fig. 109) let themost refract-ing meridian bevertical, and itsfocus be calleda, the least re-fracting meri-dian its focus b. (The astigrna- *ig. 109. tism is here re-presented as caused by altered position of the retinain different planes, instead of by altered curvatureof the cornea in different planes, the diagram being,of course, only intended to aid the comprehension * There can be little doubt from clinical observation with arefraction ophthalmoscope, that corneal As., is often still more•complicated owing to the curvature of each meridian beingpermanently and naturally more or less elliptical instead of cir-cular, and this without any tendency towards conical cornea,.as commonly 298 ERRORS OF REFRACTION A«ND ACCOMMODATION of the principle.) (1) Let a fall on the retina (lrFig. 109), and b, therefore, behind it. There is the vertical meridian, and therefore H. in thehorizontal meridian ; this is simple H. As. (2) Letb fall on the retina (2, Fig. 109), and a in front of horizontal meridian is, therefore, E., and thevertical meridian M.; simple M. As. (3) Let a andb both lie behind the retina (3, Fig. 109). There mH. in both meridians, but more in the horizontalthan the vertical meridian; compound H. As. (4)a and b are both in front of the retina (4, Fig. 109).There is M. in both meridians, but more in thevertical than the horizontal; compound M. As. (5)a is in front of the retina, and b behind it (5, Fig. 109).There is M. in the vertical and H. in the horizontalmeridian ; mixed As. The general symptoms of As. are of the same orderas those caused by the simpler defects of refraction ;but attention to the
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