By CHAS. H. HELFRICH, M.D., Surgeon to the N.Y. Ophthalmic Hospital.
Normal Refraction and Accommodation.-The dioptric media of a normal or emmetropic eye (cornea, aqueous humor, lens and vitreous humor) have the requisite refractive power to bring parallel rays of light to a focus on the layer of rods and cones of the retina. These media are centered on the optic axis, a line passing through the centre of the cornea and the posterior pole of the eye.
Upon the optic axis are situated the cardinal points of the dioptric system.
Objects situated at a distance of five metres or more are considered as being at infinity, because those rays from them which enter an eye are so slightly divergent that for practical purposes they may be considered parallel. As parallel rays are brought to a focus at the second principal focus, the eye is capable of forming distinct inverted images of distant objects upon the retina.
The eye, however, can also see near objects distinctly, and as the rays from such sources become more divergent the nearer they approach it is obvious that it must contain some mechanism to increase its refractive power. The power by which it is increased so that divergent rays are also brought to a focus on the retina is the accommodation.
By the term static refraction is meant the power the eye has when at rest (without an effort of accommodation) to bring parallel rays of light to a focus on the retina or to render divergent rays less divergent.
The dynamic refraction constitutes the increase of refractive power produced by the effort of accommodation.
THE MECHANISM OF ACCOMMODATION is as follows: By contracting the ciliary muscle the tension on the zonula of Zinn is relaxed, permitting the lens to become more convex through its own elasticity, and thus increasing the refractive power.
The changes which take place in accommodation are represented by the dotted lines in (Fig.9)
The anterior surface of the lens advances and becomes more convex, while the convexity of its posterior surface increases but little and does not change its position at all. Associated with this act is a contraction of the pupil.
THE FAR AND NEAR POINTS.-The name punctum remotum, or far point, is given to the point to which the eye is adapted when at rest. It represents the most distant point of distinct vision, and is designated by R. By the term punctum proximum or near point, is understood the nearest point of distinct vision. It is found by ascertaining the nearest point at which the smallest test letters can be read, and is designated by P. It is possible for the eye to see all objects distinctly between these two points.
THE RANGE OF AMPLITUDE OF ACCOMMODATION is the amount of accommodative effort of which an eye is capable, and is equal to the difference in the refractive power when in a state of rest and when its accommodation is exerted to the utmost. It may be represented by that convex lens, placed in front of an eye, which would give to rays coming from the near point a direction as if they came from the far point. If we consider a equals the number of dioptres represented by the range of accommodation, P the number of dioptres represented by the eye when adapted to its near point, and r the number of dioptres represented by the eye when adapted to its far point, we can calculate the amplitude of accommodation by the following formula:- a=p-r.
In the emmetropic eye R is at infinity, therefore r=0; hence a=p. To illustrate, when the near point is 20 cm. (a focal length of 20 cm. represents a lens of 5.D) from the eye, we have a=5.D.
In myopia R is at a fixed distance, and, for example, if it is situated at 50 cm. (myopia of 2.D) and P at 20 cm. (5.D) we have a=5. D.-2. D.=3. D.
The hyperopic eye is adapted for rays which converge to a point behind the retina, therefore r is negative and must be added to p. In this case we have a=p-(-r) and reduced a=p plus r. To illustrate, if the hyperopia is 10.D. and P is situated at 20 cm., we have a=5. D plus 10. D=15.D.
CONVERGENCE.-Ordinarily man looks simultaneously with both eyes, yet appreciates but a single image. This union in one single impression of the retinal images received by both eyes is called binocular vision. In order to obtain this each eye must receive upon its fovea centralis a distinct image of the object, and hence it is necessary that both lines of fixation (a line connecting the object of fixation with the centre of rotation) be directed towards the object looked at. When looking at a distant object the lines of fixation are parallel, but the nearer it approaches the more the lines of fixation must converge and the eyes turn in. If an object is moved along the median line (I M, Fig.10), a line perpendicular to the middle of a line uniting the centres of rotation, both eyes converge equally to any given situation. The degree of convergence is measured by the angle through which an eye turns when it fixes the object. When it is situated at I, one metre distant from the eye, the angle of convergence E I M is one metre angle which is taken as a unit. If the object be situated at 1/2 of a metre, it is obvious that the angle of convergence is twice as large as in the former instance; that is, it equals 2 metre angles.
ACCOMMODATION AND CONVERGENCE ASSOCIATED.-With every degree of convergence is associated a certain effort of the accommodation. When looking at an object situated at one metre, it is necessary to converge 1 metre angle, and an effort of the accommodation equal to a convex lens of 1 D. must be employed. That is, the refraction and convergence must increase by an equal quantity, which is the inverse of the distance of the object.
This association between accommodation and convergence, however, is not absolute, for with the lines of fixation fixed on a given point and stationary, the accommodation can be somewhat increased and diminished; and conversely, with a given amount of accommodation, the degree of convergence can be augmented and reduced.
If an object is held at one metre and first weak convex and then weak concave glasses be placed before the eyes the distinctness of the image is unaltered. The relative amplitude of accommodation is thus obtained. The part represented by the strongest convex glass which can be placed before the eye without affecting the distinctness of the object is termed the negative, and the part represented by the strongest concave glass the positive. When sustained efforts of the accommodation are necessary at any distance, it is essential that the positive relative amplitude of accommodation be considerable.
That the convergence may be altered while the same effort of accommodation is maintained can be demonstrated by placing a weak prism with its base in before one eye. If the convergence remained unaltered, the prism would cause double vision, but the eyes rotate outward and the object looked at is still distinct and the image single. Likewise, it will be found that a weak prism with its base out will be followed by a rotation of the eye inward with no effect on the distinctness of the image. The relative amplitude of convergence is thus obtained.
THE ANGLE ALPHA AND ANGLE GAMMA.-The optic axis A A’ (Fig 11.) is an imaginary line, which may be regarded as passing through the centre of the cornea C and the posterior pole of the eye- a point situated between the fovea and the optic papilla. Upon it are the cardinal points and the centre of rotation M.
This visual line O F unites the point of fixation O-the object looked at-with the fovea. It does not coincide with the optic axis, but crosses it at the nodal points.
The line of fixation O M joins the centre of rotation with the point of fixation.
If the fovea coincided with the posterior pole, the visual line, line of fixation and optic axis would also coincide, but this is not the case.
The apex of the corneal ellipsoid E does not coincide with the centre of the cornea, and therefore neither does the major axis of the ellipse E L coincide with the optic axis.
The angle O X E formed by the visual line and the major axis of the corneal ellipse is called the angle alpha.
When the anterior portion of the corneal axis is situated to the temporal side of the line of vision, the angle a is called positive; when it is situated to the nasal side, negative.
The angle O M A formed by the line of fixation with the optic axis is called the angle gamma.
It is termed positive when the anterior extremity of the line of fixation passes to the inner side of the optic axis, and negative when it passes to the outer side.
In practice, it is usual to consider the line of fixation and the visual line as indentical.
In order to measure the angle gamma, the patient is placed before the perimeter as for an examination of the field of vision. A lighted candle is moved along the arc of the perimeter, and by means of the corneal reflection of the flame the centre of the cornea is found. The position of the candle at the perimeter is now read from the arc in degrees and represents the size of the angle. Its average size is five degrees.
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.