Latent astigmatism may, in many cases, be made manifest and measured without a cyeloplegic, by giving lenses, which correct the manifest only, to be worn a day or two, when additional astigmatism will be manifest, which is also to be corrected; and so on, until the latent trouble is unmasked. II. M. Starkey (Trans. Sec. on Oph., Amer. Med. Assoc., '95).
In determining astigmatism make the meridian of least refraction slightly my opic, and determine the astigmatism by concave cylindrical glasses. afterward ascertaining the spherical glasses. In other words, make the astigmatism my opic, and then measure it in the remote zone. generally by distance tests. George .T. Bull (01)1). Review, p. 275, '95).
Whether astigmatism is even partially corrected by unequal contraction of dif ferent parts of the ciliary muscle, or is not so corrected, must still be regarded as uncertain.
In two medical students with normal vision the vision remained good with strong cylinders, but after the use of atropine no cylindrical lens whatever could be overcome. To prevent error the eyelids were held so that they could not press on the globe. From this experi ment it is inferred that the power of ac commodation enables the eye to over come astigmatism either by change in the curvature of the crystalline lens or by tilting it. Guilloz (Arch. d'Oph., vol. xiii, p. 676).
When the retina lies half-way between the anterior and posterior focal lines, it receives small circles of diffusion, which allow lines running in different direc tions to be seen with equal clearness. Placing before one eye fine print, and be fore the other cross-threads, when the fine print was read the threads were only seen clearly if accurately placed at the point for which the eyes were focused. When reading with eyes made astig matic, it was found that the astigmatism was corrected by bringing these smaller circles of diffusion on the retina. By extremely fine threads placed to cor respond with the meridians of astigma tism it was demonstrated that under the most favorable circumstances no compensatory action in the ciliary muscle was shown. C. Hess (von Grhfe's Archiv, Part II, '96).
Treatment.—For regular astigmatism the usual remedy is the wearing of cylin drical lenses, which should correct the full amount of the astigmatism and be worn constantly. Any case of astigma tism may be thus corrected by a convex cylindrical lens with its axis placed parallel to the meridian of greatest enr vature, or by a concave cylindrical lens with its axis placed perpendicular to this, or by two lenses of proper strengths with their axes respectively parallel to the two meridians. As may readily be demonstrated mathematically or by trial, the optical effect of any possible combination of cylindrical lenses may be produced by the proper single cylin drical lens combined with the proper spherical lens.
Javal has pointed out that sometimes the meridians of greatest curvature are not perpendicular to each other. Two cases are reported, corrected by crossed cylinders with their bases obliquely placed parallel to these oblique merid ians. lloure (Arch. d'Oph., Apr., '96).
The fact that corneal incisions change the corneal curvature has suggested their employment for the correction of astig matism.
Ten observations show the possible diminution and relative curability of astigmatism by incisions in the cornea. The change from one incision was about 1 dioptre. When the incision is not carried through the cornea so as to per mit the escape of the aqueous humor, contrary to what takes place after cata ract extraction, iridectomy, etc., there is an increase of the corneal curvature in the direction of the incision. Lucciola (Arch. d'Oph., Oct., '96).
A case is reported by Dr. Bull, of Paris, in which a complete tenotomy of the external rectos for the correction of an exoplioria produced the unexpected result of curing a progressive myopic astigmatism against the rule, or, as Dr. Bull prefers to call it, "inverse" astig matism, where the greatest curvature of the cornea is in the horizontal meridian. If this astigmatism had been measured by the retinoscope alone, the relief ob tained might reasonably have been as cribed to relaxation of an irregular spasm of the ciliary muscle, particularly as the degree of astigmatism appeared to be inconstant, but the ophthalmom eter showed that the astigmatism was produced by the difference in the curva tures of the vertical and horizontal meridians of the cornea. Three days after the operation the ophthalmometer showed that this difference in the eu•va tures of the two meridians of the cornea had disappeared, while at the same time the subjective astigmatism had gone and the vision of the eye had risen to normal. Because of this record the as tigmatism and poor vision before the operation cannot be ascribed to hysteria any more than to ciliary spasm, and the conclusion is necessary that sonic corneas have their form easily info enced by the tension of the external ocular muscles. A corollary to this is that when a surgeon has to deal with such a cornea he may be able, not only to cure an astigmatism by an operation on these muscles, but also to produce an astigmatism or increase one already present by an incautious interference.
Dr. Bull's observation opens the way to investigations which may prove of essential service in some obscure con ditions. If an etiological connection between progressive astigmatism, or the position and relative tension of the ocular muscles, and glaucoma can finally be demonstrated, he will be entitled to the credit at least of having made the first suggestion, and it is to be hoped that the results of the investigations he proposes to make may be as brilliant as the result he obtained from the operation he has described. At the same time it is to be hoped that this opera tion will be tried conservatively and its limitations clearly defined before it is brought into general use, for "such an operation for the cure of astigmatism, should be undertaken only in very ex ceptional cases," while the damage which may be done by incautious or unskilled interference with the muscles of the eye has been demonstrated to be very great. Editorial (New Yot.i. Medical Journal. Feb. 7, 1903).