and the other myopic. If the retina be at lines 3, 4, or 5 (Fig. 32), the vertical rays will focus in front and the horizontal rays will focus behind, and the images 3, 4, or 5 in figure 33 are formed.

Astigmatism with the rule is a term given to cases of hyperopic astigmatism with the axis in or near the vertical meridian, and to cases of myopic astigmatism with the axis in or near the horizontal meridian.

Astigmatism against the rule is the reverse of foregoing condi

tion.

Symmetric astigmatism is the usual form in which the axes of the two eyes added together make exactly 180; as, for instance, R. axis 30, L. axis 150; or, R. axis 180, L. axis 180 (in this case one of the axes in the left is taken as o).

Unsymmetric astigmatism is the troublesome form in which the two axes collectively do not make 180; as, for instance, R. axis 130, L. axis 40; or, L. axis 180, R. axis 140. This form is a prolific cause of asthenopic symptoms.

Disadvantages.—An astigmatic eye sees a point as a line; hence images are distorted and marked visual disturbance results. It is believed that there is an involuntary effort on the part of the ciliary muscle to remedy the astigmatic defect. By constant unequal contraction this muscle partly compensates for moderate inequalities in the curvatures of the cornea. Such continuous and unequal accommodative effort must result in eye-strain, and consume much of the normal nervous energy, eventually producing reflex manifestations of disturbance of the nervous equilibrium. Therefore, an astigmatic person is able to improve his vision only at the expense of his health. Uncorrected astigmatism, particularly in delicate and nervous women, results in the worst forms of reflex asthenopic symptoms.

Symptoms. Nearly every person consulting us for refraction is more or less astigmatic, and in the majority of cases part of the astigmatism is masked by the accommodative effort of the ciliary muscle. Under this natural correction the visual acuity is more or less undisturbed, and in the moderate degrees of astigmatism the reflex symptoms are the most prominent signs. The patient complains of headache, nausea, anorexia, indigestion, inability to perform continuous near work, etc. In reading, the letters often fade out and seem to run together. There is significant confusion of such letters. Commonly there will be noticed an inclination of the head to one side, the patient saying that he can see better with his head in that position. In very high degrees of astigmatism the pupil may be oval, and the asymmetry of the cornea may be detected by simple inspection.

9

From the preceding remarks as to the involuntary correcting action of the ciliary muscle, the use of a mydriatic in determining the full degree and correct axis of the astigmatism is necessary.

Tests for astigmatism are very numerous; there are a dozen or more valuable methods of qualitative and quantitative diagnosis of astigmatism.

3

The astigmatic cards and dials and the keratoscope furnish us easy methods for simple diagnosis, while the ophthalmoscope, ophthalmometer, retinoscope, and test-lenses are valuable methods for the more exact determination of the kind and degree of astigmatism. Besides these tests we have the stenopaic slit, the perforated chimney disc, the cobalt glass, etc.

The quickest method for detecting astigmatism is the application of a low degree cylinder before the eye, which if preferred in any special axis gives a strong reason to suspect astigmatism in that axis or at right angles to it.

The diagnosis of astigmatism by the ophthalmoscope, by the retinoscope, and by the test-lenses, has been described in the sections dealing with these special instruments, and in the following lines we will only attempt to describe instruments especially designed for the detection of astigmatism.

02

Astigmatic cards and charts are of several varieties. Snellen's cards consist of a series of equally colored dark lines arranged like the spokes of a wheel, with the angle of inclination of each line marked opposite to it. Seated at six meters and looking at this card, an astigmatic person sees clearest || the lines corresponding to the axis of his astigm matism, and sees poorest the lines at right angles to this axis. Another form is the arrangement of lines similar to a clock-dial; the numbers of the clock are used to indicate the chief meridians, as these are so well known by the patient and can be easily indicated by him to the physician. (Fig. 34.)

གག་

FIG. 35.-PRAY'S ASTIG-
MATIC LETters.

Pray's astigmatic letters (Fig. 35) are formed of equally black lines inclined at different angles. The letter seen the blackest is the one formed of lines at an angle corresponding to the axis of the patient's astigmatism. Many other forms of cards and charts have been suggested.

A keratoscope is an instrument for examining the cornea and testing the symmetry of its meridians of curvature. Several forms have been devised, such as the Wecker-Masselon (Fig. 36), which is a black board 18 cm. square, bordered by a white stripe about 15 mm. broad. Through a hole in the center the image of the white frame is seen reflected on the cornea. By turning the board on its handle the white frame will take a position from which a rectangular image is reflected. When this position is found, we have the direction of the principal meridians in an

astigmatic cornea. If the mirror is held in any other direction, a rhomboid image of the retina is seen. The degree of astigmatism is found by comparing the size and shape of the image with a series of rectangles printed on an accompanying card (Fig. 37), showing the appearance of the corneal image in astigmatic conditions from o. to Io. D.

Placido's disc is a white disc about ten inches in diameter, upon the surface of which are painted concentric black rings one inch apart. The observer looks through a hole in the center of this disc and notes the appearance of the image of the rings that is reflected upon the patient's cornea. The image is elliptic or distorted according to whether the astigmatism is regular or irregular. In emmetropia the rings appear circular.

A ready method of keratoscopy is to place the patient in front of a window having several panes of glass in it, and observing the appearance of the image of the sash on the patient's cornea. The lines are curved or

irregular if astigmatism is present.

The ophthalmometer (Fig. 38) is an instrument formerly much used for the determination of the kind and amount of corneal astigmatism. It is less exact than the retinoscope, and, moreover, it is very expensive. That of Javal and Schiötz consists of a telescope attached to a graduated arc, upon which are two objects called mires, the left one being fixed, while the right is movable. These mires are white enamel, one quadrilateral in shape and the other the same size, except that on one side it is cut out into five steps.

FIG. 38.-THE OPHTHALMOMETER OF JAVAL AND SCHIÖTZ,

A. The Perimeter-bar. B, B. Telescope. C. Chin-rest. D. Disc of radiating lines and concentric circles. E. Eye-piece, at which the observer sits. H. Head-rest. L. Lights. M, M. The mires, or targets. P. Pointer, indicating the axis by the degree numbers on the peripheral border of the disc.