Abstract
IT is well known that in the normal eye, with its accomodation relaxed, parallel rays of light, that, those from distant objects, are brought to a focus on the retina. Rays from near objects are divergent, and if they enter such an eye they are not brought to a focus on the retina, but would be at some point behind it. In order that they may be so brought to a focus and form a distinct image on the retina, an effort of accommodation is necessary. This is performed by a small muscle called the ciliary muscle, inside the eyeball, the ultimate effect of whose contraction is an alteration in the shape and perhaps the condition of the lens, which causes the rays to be more strongly refracted, and brings them to a focus on the retina. The effect is in fact the same as if a convex lens were added to the optical system of the eye. As age advances, the muscle and lens become stiffer, and work with difficulty. They are relieved of part of their work by putting a convex glass in front of the eye. Hypermetropia is a condition in which the axis of the eyeball is too short, compared with the refracting power of the lens. In it an effort of accommodation is necessary to see even distant objects clearly, and a still stronger effort to see near objects. A person suffering from it requires convex glasses. When both eyes are used together, the optic axes of both are directed to the object, so that in looking at a distant object they are directed parallel, and in looking at a near one they converge. These movements are effected by the external muscles of the eyeball, which are supplied by branches of the same nerve as the ciliary muscle. As a fact these movements of the ciliary muscle and of the external muscles of the eyeball are associated, or habitually performed in conjunction; that is, the brain has become accustomed to send an impulse to the one set of muscles proportionate to that sent by the other. Any disturbance of this association can only be accomplished by a distinct effort which, if severe or long continued, is apt to be painful. Suppose a man has become presbyopic, i.e. his accommodation has gradually become stiff, and its range reduced. In order to accommodate for rays from an object at the ordinary reading distance of ten or twelve inches, he has now to exert an effort equal perhaps to what he would have employed when young on one four inches off, but the change has been gradual, and the convergencs of the eyes for twelve inches has become associated with this amount of effort. If he now use convex glasses of suitable power, the want of refracting power is supplied, the effort of accommodation is reduced to its natural amount, but the amount of convergence which has become associated with this small effort is now insufficient, and the eyes, instead of converging to twelve inches, converge on a point several feet distant, so that double vision would be produced, unless by a distinct effort the eyes were converged more, and this effort is often painful, and is expressed by the term that ihe spectacles “draw” the eyes. After a time new associations are formed, and the spectacles can be used comfortably; but this does not happen in all cases, and for these it is necessary to grind the lenses on glasses of prismatic section. The action of the prism is so to bend the pencils of rays coming to the eyes that they appear to diverge from a point corresponding to the new focal distance of the eyes provided with the spectacles. Sometimes the amount of prismatic effect required is calculated, but the calculation, being based on general considerations, does not always suit individual persons; at other times prismatic glasses from a trial case, are combined with the calculated spherical, or spherical and cylindrical glasses, until one is found with which vision is comfortable. In many cases it is not necessary to use glasses specially ground on prisms, but sufficient to move the centre of the glasses nearer together. The glass being thicker in the centre, looking through the part near the edge produces an amount of prismatic effect which is often sufficient. If concave glasses are used, as in cases of short sight, then they must be further apart than the distance of the eyes, in in order to produce this effect. The object of the instrument exhibited is to find experimentally the amount of prismatic power, and the distance of the centre of the lenses which isrequired in any individual case. Two circular frames each 2½ inches in diameter, and with teeth cut in their edges, are mounted, so that the teeth gear into each other, and they can rotate freely, but in opposite directions. In the centre of each frame is mounted a prism of 18° one of the frames is graduated, and when the graduation is at 0° the axes of the prisms are parallel, so that parallel pencils of rays falling on both are deviated both in the same direction, and still parallel. Thus when the pair of prisms are arranged horizontally in front of a pair of eyes, an object looked at appears displaced up or down, but there is no lateral deviation on either. If the frames be rotated 90° in one direction, the prisms both have their bases inwards, or, if in the other direction, both outwards, so that two pencils of rays are deviated to the full power of the prisms. In the intermediate positions part of the prismatic effect is resolved in a direction at right angles to the line joining the centres of the frames, and can be neglected as only producing parallel displacement of the image, and part is resolved in the direction of this line so as to produce apparent separation or approximation of the images. This amount is read off from the graduation, which is constructed on the following principle:—Suppose a ray of light X A O perpendicular to the plane of the paper meets the paper at O. Suppose a prism be introduced at A having an angle of deflection θ, the ray of light now falls on the paper at B. If the prism be rotated through angle β, the ray now falls on the paper at C. Join O B, O C, and resolve O C into vertical and horizontal co-ordinates C D, O D. C D being neglected as described, we wish to find O D the horizontal component of the deflection.
Article PDF
Rights and permissions
About this article
Cite this article
A Prismatic Optometer 1 . Nature 24, 618–619 (1881). https://doi.org/10.1038/024618a0
Issue Date:
DOI: https://doi.org/10.1038/024618a0