different meridians, ie. of corneal astigmatism. In Javal's instrument, while the one object is rectangular, the other is so shaped, in steps, that for every step overlapping in the meridian of greatest curvature there corresponds one dioptre of astigmatism. This is shown in Fig. 56, which represents four dioptres of corneal astigmatism."

test.

This instrument has the merit of being an objective The indications which it gives are definite and rapidly obtained, but opinions are considerably at

variance as to its real value for accurate work. It has, moreover, the disadvantage of being expensive.

PLACIDO'S DISC is another method of ascertaining the presence of astigmatism. It only, however, indicates the axes of greatest and least curvature, but this is seen at once. It consists of a metal plate about IO in. in diameter, on which are painted a number of concentric circular white and black bands, arranged alternately and about half an inch in breadth. A hole is placed in the centre, this contains a plus glass of about 4 D, and from it extends a short tube. The manner of using it is as follows:-Place the patient with his back to the window, or, in a dark room, an Argand burner behind him. If the observer now

with

looks down the short tube and holds the disc a little distance from the eye to be examined, he will notice an image of the concentric rings on the cornea. If there be no astigmatism, the rings will be circular. On the other hand, should there be astigmatism, the figure will be elliptical in shape, the long axis corresponding with the meridian of least curvature, and the

short axis of the ellipse with the meridian of greatest curvature of the cornea. In irregular astigmatism the image of the rings on the cornea will be irregular and distorted.

The author has employed a disc perforated in the centre and marked with rays across the circle, like a double Snellen's astigmatic fan, with, however, fewer radii (p. 19). If this is advanced towards the patient, the reflection of the lines will be observed on the patient's cornea, and should astigmatism be present, one of the

radii will become distinct and sharp before the others, and the direction of one axis is at once rendered evident. A slit is provided behind the perforation, in the centre of the disc, for placing a convex glass (+3 D) before the observer's eye; and the disc is so made that the front plate with the radiating lines can be rotated. A single line running across a circle may be used, and by watching the reflection on the cornea, as the disc is rotated, the direction can be noted in which the line is most sharply seen. White lines on a black ground are to be preferred.

Having described the methods for investigating cases of refraction, it will be well now to mention the routine plan to be adopted in any individual case.

1. Ophthalmoscope. The nature of the error will be discovered by standing with the mirror at a short distance from the patient, as described on p. 47, or the degree of ametropia may be measured by the direct method. The accuracy with which this measurement can be done will very much depend upon the skill of the observer. Latent hypermetropia will frequently be overlooked. Until, therefore, the observer has gained considerable experience, he will do well, after having ascertained that the media are clear, and that the ophthalmoscope reveals no change in the fundus, to note the kind of ametropia present, and to proceed at once to the examination by the shadow test.

2. Retinoscopy. This is the most easily acquired, as well as one of the most reliable methods of investi

gation. For children it is indispensable. The observer is therefore advised to rely upon it for his objective examination. The axes of the meridians in astigmatism are easily detected by this method, and a beginner will find it of service at a time when he is compelled to rely more upon subjective investigation with test types than, with greater experience, will be necessary.

3. The results obtained by the shadow test must be verified by trial glasses and Snellen's types. No examination is complete without the use of the test types and trial glasses, but the whole investigation will be considerably shortened by the previous use of the shadow test.

A Committee of the American Medical Association, appointed to report on the value of the objective tests for the determination of ametropia, reported as follows:

Ophthalmoscopy.—The ophthalmoscope enabled one to immediately assign an approximately just share to the influence of ametropia in causing imperfect vision, and that the presence and degree of hypermetropia, myopia, and astigmatism were ascertained very quickly. As to the ability of the ophthalmoscopic examination to reveal latent hypermetropia there was some doubt, as it failed in a certain portion of cases. Accuracy with this method cannot be obtained by all observers, or by any observer, at all times and in all cases.

Ophthalmometry.-Ophthalmometry determined the existence and amount of corneal astigmatism to

within .25 D, or less, if the patient be steady and the other conditions favourable. The ophthalmometer gave no information as to the degree of hypermetropia or myopia present, and the committee thought that the chances of conformity between the corneal and total astigmatism are not sufficient to justify reliance upon the method as determining the cylindrical correction required.

Retinoscopy was, upon the whole, the most accurate and reliable objective method of estimating ametropia. Applied in the direction of the macula, it measures in the visual zone exactly the refraction that is determined by the subjective method.

The Committee urged that while each of these methods should be used, they should be followed by the subjective determination.