Eye disorders > Determining eye refraction > Acuteness of vision

Acuteness of vision

The Acuteness of Vision - This must not be confused with the refraction, and it is necessary clearly to understand the difference between them. The acuteness o£ vision is the function of the nervous apparatus of the eye, while the refraction is the function of the dioptric system; so that the acuteness of vision may be normal, even if the refraction be very defective, provided it has been corrected by glasses. The refraction, on the other band, may be normal, even though the eye is unable to see, as in cases of optic atrophy. We may define the acuteness of visio'n as that degree of sight which an eye possesses after any error of its refraction has been corrected, and the glasses necessary for this correction are therefore a measure of the error of refraction.

In order to find out the acuteness of vision, we have to determine the smallest retinal image, the form of which can be distinguished; it has been discovered by experiments that the smallest distance between two points on the, retina which can be separately perceived
is 0.00436 mm., about twice the diameter of a single cone; but it is only at the macula and the part immediately around it, which is the most sensitive part of the retina, that the cones are so close together as .002 mm.; in the periphery of the field of vision the two
points must be further apart to appear distinct.

It is probable that rays from two points must fall upon two different cones in order to be visible as two distinct points.

The smallest retinal iinage which can be perceived at the macula corresponds to a visual angle of 1', so that two stars separated by an angular interval of less than 1' would produce upon the eye the effect of one star only.

The visual angle has been shown to be an angle included between two lines drawn from the two opposite edges of the object through the nodal point (Figs. 28 and 35).

Test-types have been constructed upon these principles for determining the acuteness of vision, Snellen's being those ordinarily used. Every letter is made so that when at its proper distance, each part of it is separated from the other parts by an interval equal to not less than the arc-subtending an angle of 1' at
the nodal point, while the whole letter subtends an angle of 5'.

In order to estimate the refraction by the acuteness of vision, the test object must be placed in a good light, and so far away as to exclude as much as possible the accommodation, C, metres has been found to be a sufficient distance; rays coming from an object
so far off may be assumed to be parallel, and falling on an emmetropic eye at rest, would come to a focus on the retina. The smallest letter which can be seen distinctly at this distance will represent the patient's vision.

Snellen's type consists of rows of letters, each being marked above with the distance in metres at which it should be read. The top letter should be distinct at 60 metres, the next at 36, and each succeeding row at 24, 18, 12, 9, and 6 metres respectively. The patient placed at 6 metres should, without any accommodation, be able to read the bottom line with either eye. This is expressed in the form of a fraction, in which the numerator is the distance at which it is read, and the denominator the number of the line. We note down the result found for each eye separately : if the
bottom line is read, 6/6 expresses it; if the next, 6/9 ; the
top, 6/60, and so on.

If our patient, however, be not able even to see the large letter at the top, we allow him to approach the board, telling him to stop as soon as the letter becomes visible. Supposing he stop at 2 metres from the board, we express that as 2/60 ; if he is not able to read it at all, we see how far off he can count fingers.
If unable to do this, a lower degree of visual acuteness is found out by determining the ability to distinguish different sorts of light, as to colour. This is called qualitative perception of light whereas a still lower degree is to distinguish the difference between light and darkness; this is "quantitative perception of light."

Although the capability of reading the bottom line at 6 metres is the average of acuteness at different times, yet it is not the maximum, since many young people will be found who are able to read line six at 7 metres, or even further, in which case their acute-
ness is 7/6. "Savages" often have an acuteness of vision much in excess of the normal.

Thus we have a standard of normal vision, and a convenient method of expressing it in a numerical manner. We put our patient then, with his back to the light; in front of the test-types, which must hang well illuminated at 6 metres distance, and having armed him with a pair of trial frames, we exclude the left eye from vision by placing in front of it a ground glass disc, and praceed to test the right eye by asking him how much of the type he is able to read; if he read the line marked 6, then his vision is 6/6 or 1, that is to say, his distant vision is normal; we may, therefore, assume the absence of myopia or astigmatism ; but he may have hypermetropia, and only be able to read by using his accommodation; this we decide by
holding a weak convex glass ( + .5 D.) in front of the eye, when, if he still be able to read the same line 6/6 he has hypermetropia, and the strongest convex glass with which 6/6 can be read is the measure of the manifest hypermetropia; supposing + 1 D. the strongest glass with which can be read, then we record it thus: R.V. 6/6 Hm. 1 D. = 6/6.

I say manifest hypermetropia, because in all cases occurring in young people this is not the total hypermetropia; for in these a great part is latent, which can only be discovered by using atropine, or by estimating the refraction by the direct ophthalmoscopic method. Many cases will come before us having
two or three dioptres of hypermetropia, who complain that the weakest convex glass impairs distant vision; in those cases the hypermetropia is wholly latent.

We may say, therefore, that a patient who is able to read ; with one eye, must be emmetropic or Hypermetropic, in that eye.

If hypermetropic, a part of it is usually manifest, as found out by the strongest convex glass which does not impair distant vision; or it may be wholly latent, when it is necessary to atropise the patient before we can demonstrate it.

Supposing, however, our patient's vision is below like normal, and, instead of reading 6/6, he is only able to read, say the third line (6/24), and that this is blurred with a weak convex glass, he may have Myopia, Astigmatism, or Spasm of accommodation.

We try if a weak concave glass helps him; if it does so, the case is one of myopia; and we find the weakest concave glass with which he sees best; thus we take an example in which the patient is a myope and sees only 6/24, but with -2 D. 6/6; we repeat the examination
with the second eye, and record it accordingly.

R.V. 6/24 - 2 D. = 6/6
L.V. 6/24 - 2 D. = 6/6

If our patient is not improved with concave glasses, then we assume that some astigmatism is present, presupposing of course that there is no other cause for bad vision.

To estimate this astigmatism we must call to our aid some of the methods described in astigmatism section, or we may find out the spherical glass with which lie is able to see best, then rotate in
front of it a weak convex cylindrical glass; no im-provement occurring we do the same with a weak concave, cylinder, finding thereby the glass and its particular axis, wbich gives the best result. It is necessary that the eye be thoroughly under the influence of atropine, in oraer to enable us to arrive at definite and reliable results by this method. With practice, one is able in this way to work out simple cases of astigmatism accurately ana quickly.

The object in view is always to bring up the vision of each eye as nearly to the normal standard of 6/6 as possible. Frequently, however, we have to be satisfied with 6/9 or 6/12.

But should the case appear to be a difficult one, it, is better perhaps for the student not to waste time, but proceed at once to retinoscopy.

When trying the patient at the distant type it is often convenient to have two sets of letters on the opposite sides of the same board, so that it may be reversed when the patient gets too much accustomed to the letters on one side.

The near typo is chiefly used to estimate the acommodation, by finding out the far and near point at which any particular line is read. Snellen's and Jaeger's are the types most commonly in use, many preferring Jaeger's, owing to the letters being of the ordinary shapes, but they have the disadvantage that they are not arranged on any scientific plan, but are simply printer's types of various sizes: the set of reading type at the end of the book is so arranged
that when held at the distance for which it is marked, each letter subtends an angle of 5' at the nodal point. It must, however, be remembered that sentences are an inferior test to letters, many people recognising the words by their general appearance, whereas they would be unable to see distinctly each letter of which
the sentence was composed.
Having tested our patient's vision at the distant type and recorded the result, we place in his hand the reading type, and note the smallest type he is able to read and the distance at which he, reads it; first with each eye sepai'ately, then with the two together.

In cases of myopia we may thus get a valuable hint as to the amount of the defect.; we will take for an example a case in which the patient can read 6/24 with the right eye; we give him the wear type, and if he can read the smallest, only by holding it at a nearer point than the distance for which it is marked, we note the greatest distance at which he is able to read it; if the type marked for 1 metre cannot be read further off than 25 cm., our patient has then most likely myopia of 4 D., because 25 cm, is probably his far point. In this case a glass -4 D. would give to rays coming from a distant point the same amount of divergence as if they came from 25 cm. (100/25=4).

We try the patient at the distant type with -4 D.; if he now read 6/6 the myopia is confirmed, and the weakest glass with which he reads it is the measure of his myopia.

If the patient read 6/6, but be unable to read the near type, except it be held at a further distance than that for which it is marked, the case is one of paralysis of the accommodation, or presbyopia ; and as the latter only commences in emmetropia about the age of fortyfive, it will be clear according to the age of the patient
to which division the case belongs.

As objects seen through convex glasses appear enlarged, and through concave glasses diminished, it follows that these, when placed before the eye, will exercise the same influence on the size of the retical image. Now the hypermetropic eye sees objects dmaller, and the myopic eye larger than the emmetrope, and if glasses which are to correct the ametropia be placed at the anterior focal paint, i. e. about 13 mm. in front of the cornea, the fetinal image of the ametrope, should be of the same size as that of the emmetrope.

Before leaving this subject of the acuteness of vision the following directions may be given:

lst. The test-type must be in a good light; the advantage of artificial illumination is that it is uniform.

2nd. Commence with the right eye, or that which has the best vision, covering up the, other with an opaque disc placed in a spectacle frame; do not be contented to allow the patient to close one eye, as he may not do so completely, or he, will probably unconsciously slightly diminish the palpebral aperture of the eye under examination, whereby the circles of diffusion may be somewhat diminished and so give misleading results. Neither should he close the eye with his hand, he may look between the fingers, or exercise some pressure, however slight, on the eyeball, which may interfere temporarily with the function of the retina and so cause delay.

3rd. Having noticed what each eye sees without glasses, always begin the examination with convex ones, so as to avoid calling the accommodation into action.

4th. Having noted the result found for each eye separately, we try the two together, the binoculair, visual acuteness being usually slightly greater than that for one eye.

5th. Test the patient with the reading type, noting
the farthest point at which the smallest type can be
read.