Fogging is next resorted to by producing artificial myopia, which encourages relaxation of the accommodation. The thumbscrew is now tuned slowly back and the patient requested to watch the astigmatic fan. If he states that one or more lines appear distinct before the others he is astigmatic, whereas if they appear equally distinct simultaneously the error is simple hyperopia or myopia. If astigmatism is present it is necessary to utilize the concave cylinders contained in the revolving discs to render the lines equally clear. This procedure should be repeated to verify the result or to make any necessary corrections.
The power of convergence is tested with an ophthalmo- dynamometer, that of Landolt’s being the simplest and best. It consists of a metallic cylinder blackened on the outside, containing a vertical slit 0.3 mm. wide covered with ground glass. Beneath the cylinder is attached a tape measure graduated on one side in centimetres, and on the other in metre-angles. The vertical line of light, produced when the cylinder is placed over a lighted candle, is the object of fixation. Approaching the patient in the median line until the patient sees the line double the near point of convergence is found and the distance in centimetres with its equivalent in metre-angles recorded. The minimum of convergence is found by withdrawing the instrument from the patient; but as it is usually negative, it is determined by the strength of the strongest abducting prism which will not cause diplopia with the patient fixing the object at six metres. If the number of this prism is divided by seven, the quotient will approximately give in metre angles the deviation of each eye when the prism is placed before one. The amplitude of convergence is equivalent to the difference between the maximum and minimum of convergence.
Accommodation is tested by means of the reading types of Snellen or Jaeger. A sample is shown in Fig. 24.
It is necessary to test each eye separately, and finally both together. The nearest point the type can be read represents the punctum proximum. This subject has already been discussed in the preceding chapter.
Presbyopia is to be determined after the static refraction has been tested. With the distance glasses in the frame, the patient is asked to hold the reading type or a newspaper at the distance at which he desires to work or read. Convex glasses are now added to the distance glasses, until the best vision at this distance is obtained. The optical equivalent of the glasses before each eye is now computed, and the resultant glass is presumably his prescription for near. The approximate amount of presbyopia at different ages can be computed from the chart (Fig.10) given in the previous chapter. This amount must be added to the amount of hyperopia and deduced from myopia. This should not be considered as final, but used simply as a check on the result obtained by testing and to save unnecessary loss of time.
OBJECTIVE DIOPTOMETRY.- Objective dioptometry embraces the methods of determining the refraction independent of any statements by the patient. These methods are valuable in conjunction with the test made by the trial lenses and test letters, and especially so when the patient is illiterate or too young to read letters.
Frequently they are utilized to save time in arriving at an approximate estimate of the error, the accurate degree of which is fully determined in the ordinary manner. Not that they are inaccurate in themselves when applied by an expert, but because it is a safer plan to check results.
Estimation of Refraction by the Direct Method- A qualitative estimation can be made with the ophthalmoscope held at a little distance from the observed eye. When it is remembered that the rays issuing from a hyperopic eye are divergent and those from a myopic eye convergent, it is easy to understand how an observer can see an upright image of the retinal vessels in the former and an inverted aerial image in front of the latter. If the observer moves his head from side to side, the vessels seen in a hyperopic eye will move with the mirror, the image being upright, while they will move in the opposite direction if the eye be myopic. At this distance no vessels can be seen in an emmetropic eye, because the pencils of rays emanating from any two points upon the retina (each is made up of parallel rays) will diverge from each other so that no rays will enter the observer’s eye. Close to the eye, however, an upright image of the fundus can be seen. The quantitative examination is conducted with the mirror held close to the observed eye, if possible as near 13 mm., the anterior focus of the eye and the proper situation for the glasses to be worn. If the examination is conducted at a greater distance proper allowance must be made. In following out this method, it is imperative that both the surgeon and the patient thoroughly relax their accommodation. This is easily accomplished for the patient, if no spasm exists, when the examination is made in a dark room, but the examiner can only attain it after much practice. It is rather doubtful if any expert can relax his accommodation absolutely unless he be so old that he practically has none. Still, many can do so thoroughly enough to obtain approximately correct tests.
The most desirable point in the eye-ground upon which to focus is either the edge of the disc or the medium-sized vessels between the disc and the macula, especially two vessels running at right angles to each other. The macula is unsuitable, because of the contraction of the pupil caused by throwing the light upon it and the annoying corneal reflections obtained.
If the observer has any error of refraction, it must either be corrected with glasses or allowance made for it in computing the final result. all these conditions being fulfilled the examination is commenced. If the patient’s eye is emmetropic, the vessels will be seen distinctly, and convex lenses rotated back of the opening in the ophthalmoscope will blur them. If hyperopia is present the divergent rays issuing from the eye are rendered parallel by the rotation of convex lenses, and the strongest convex lens through which the vessels are seen distinctly is the measure of the error. The convergent rays from a myopic eye are rendered parallel by concave glasses, and the weakest concave lens through which the vessels are seen most distinctly is a measure of the myopia. The direct method is used to determine the height of retinal tumors by estimating the refraction at their summit, and the depth of a cupping of the papilla of estimating the refraction at its bottom.
If the examination is conducted at a distance not more than 2.5 cm. from an eye, a hyperopia of 1. D will represent a shortening of the axis equal to 0.3 mm. and a myopia of 1.D a lengthening of the axis to that same amount.
Astigmatism can also be estimated by this method. It is known to be present when the upper and lower margins of the disc and the horizontal vessels are well defined while the lateral margins and vertical vessels are blurred, or vice versa.
Its presence may also be suspected if the disc is elongated either horizontally, or vertically, the long axis corresponding to the meridian of greatest refraction.
In estimating the degree it is the best to fix tow vessels running at right angles to each other and whose direction conform most nearly to the principal meridians of the astigmatism. If the vessels in one meridian are seen distinctly, while in order to see the vessels in the opposite meridian a convex or concave lens is necessary, the case is respectively one of simple hyperopic or myopic astigmatism. If, in order to see distinctly the vessels in the two principal meridians, two convex or two concave lenses of different strength is required, the case is one of compound astigmatism, either hyperopic or myopic. The difference in strength between the two convex or two concave lenses represents the astigmatism, while the weaker lens represents the simple hyperopia or myopia. The axis of the cylinder to correct the astigmatism is placed in the direction of the vessel which was seen through the strongest of the two lenses.
It seems unnecessary to again state that in hyperopic astigmatism the strongest convex lenses represent the measure and in myopic astigmatism the weakest concave.
In mixed astigmatism the vessels in one meridian are rendered distinct by convex glasses and the vessels in the opposite meridian by concave glasses. Hyperopia exists in the meridian at right angles to that in which the vessels were made distinct by the convex glasses and myopia exists in the other. The axes of the cylinders to correct this would be the reverse of this, because they refract only those rays which enter at right angles to their axes.
Estimation of the Refraction by the Indirect Method.- This method is not generally resorted to, because of its difficulties. It is sometimes used in estimating very high degrees of myopia, but rarely in hyperopia. The rays coming from an emmetropic eye being parallel they are brought to a focus by the object glass at its principal focus, whereas the divergent rays from a hyperopic eye are brought to a focus farther from the object glass than its principal focus and the convergent rays from a myopic eye nearer than its principal focus. The degree of the ametropia is determined by measuring this distance. Schmidt-Rimpler’s method is usually employed.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.