The Vision” Of the Pupils of a1st Elementary School Tested by the Snellen Alphabet and Illiterate Cards
- Author:
McCaulie,
Principal of the Centennial School, Trenton, N. J.
In 1862 Dr Herman Snellen, professor of ophthalmology in the University of Utrecht, Holland, and director of the Netherlands Institute for Diseases of the Eye, published an improved series of test types for measuring the acuity of vision. The principle which guided him in the construction of these test letters is based upon the fact that the normal eye can just discern objects that subtend a one minute angle, the vertex of the angle being the point where the rays of light cross before falling on the retina.
In order that a letter may be recognized, each one of its elements must be discernible, and hence each of these elements must have a diameter equal to the tangent of an angle of at least one minute. In constructing uniform letters in conformity to this principle, Snellen found that each letter must have at least one diameter equal to the tangent of a five minute angle. Each letter is therefore made in a square which is subdivided into twenty-five equal squares, each small square being equal in diameter to the tangent of a one minute angle. Since the tangent of a five minute angle is equal to 0.001454, to obtain the longest diameter of a letter to be seen at a given distance, Snellen multiplied the length of the tangent of five minutes by the distance (in centimeters) of the letter from the nodal point of the eye; thus, at a distance of one hundred centimeters the height of the letters should be 0.1454 centimeter, and each stroke of the letter should be at least one-fifth of this, or 0.0291 centimeter in length. The tests described in this paper were made during April and May, 1907, upon the children enrolled in the Centennial School, Trenton, K”. J. Although made primarily, of course, with the object of ascertaining the number of pupils whose vision was subnormal, the examination also yielded data for a comparison of the results obtained by using Snellen’s alphabet with those obtained by using his “illiterate E.”
Knowing tliat the scientific value of the work which I was undertaking depended largely upon the accuracy of the methods adopted, I first secured a copy of Snellen’s classic work on test types.* This book contains eight letters,?0, L, UST, Z, B, D, T, and E, and also the “illiterate E,” pointing in four directions,? up, down, right, left. (See figure 1, p. 181.) The characters are all of a size and structure to be read by the normal eye at sixteen (Paris) feet,f and are printed on white unglazed paper, about the thickness of ordinary book paper.
Each one of these letters I carefully cut out, leaving around each a margin of about one-fourth of an inch of white paper. The letters were then pasted on white cards, three and one-half by six inches, being placed one and one-half inches from the top of the card and equally distant from the sides. On the back of the card was written the same letter that appeared on the front. I selected letters that could all be read at one distance, rather than letters of different size, to be read at different distances, because the results could be more easily compared. By having the letters on separate cards, so that only one was in view at a time, I could be sure that the pupil’s reply was a judgment on that letter and not on some other in the same line, as often happens when several letters are shown at one time. I was likewise enabled to vary the order of presentation, and so prevent their being memorized; and, by glancing at the back of the card, I knew each letter before the pupil saw it. This plan left me free to give my entire attention to the efforts put forth by the children in reading the letters. By holding the cards in my hands T was able to utilize the best light in the room more easily than if I had used a large card hung on the wall, as is usually done in such tests.
Next I selected the best position for light in each room where the tests were to be made. Beginning with this position, I made chalk marks on the floor the entire length of the room, the first mark being two feet away from the card, the second four, and so on. All of these tests were made in the class rooms while the teacher was carrying on the regular work.
Everything being ready, the pupils were called one at a time and told to stand with their toes to line twenty, with a card held over one eye. Two or three letters were presented. If the pupil could not read them, he was asked to step up two feet and try again. ?Snellen, H. Optotypl ad visum determlnandum secundum formulnrum v = Ed. XVIT. (Probebuehstahen 7,ur Bestimmung der Sehschilrfe.) Berlin, II. Peters. 1904.
fThe Paris foot is a little more than twelve and three-quarters inches of the English scale. THE VISION OF PUPILS. 1?? If he failed again, lie was asked to step up two more, and so on until he was able to read each letter correctly with little or no hesitation. If he could read all the letters correctly at sixteen feet, his vision was considered normal and this fact was indicated by marking the fraction ff opposite his name in the records. If he could read the letters only at six feet, this fact was indicated by the fraction TrV . Such an eye, of course, would be subnormal and probably near sighted. If the pupil could read the letters at twenty-four feet, this fact was represented by the fraction showing that the eye was subnormal and far sighted. Having determined the distance at which one eye could read the letters, I repeated the same process with the other eye.
Immediately after each pupil was tested with the alphabet cards, he was tested with the “illiterate E” cards by the same general method, except that instead of naming the character the pupil was required to point in the same direction,?up, down, right, or left,?that the E pointed. In this case, as with the letters, a single error was taken to indicate that the pupil’s vision was not sufficiently acute to read the characters at that particular distance, and he was required to move up until he reached the point at which every character could be read correctly.
The time taken by the pupils to recognize the letters varied considerably. It took some pupils three or four seconds to recognize some of the letters, but the average time for each letter was a little over a second. A pupil was allowed to study over one letter three or four seconds, only in case he named the others correctly and rapidly. Some pupils recognized the letters in one-third the time taken by others in the same grade. As a rule, the pupils of the lower grades responded much more slowly than did the pupils of the higher grades. I often found it necessary in all the grades, especially in the primary grades, to allow the pupils to rest their eyes. Many eyes after trying to read half a dozen letters would be filled with tears, which probably indicated weakness of the ocular muscles.
Before beginning the test each day, I tried the cards myself to see whether the light was satisfactory. Usually the best light for presenting the letters was found at the rear of the room. This position prevented the pupils in the room from seeing the letters and becoming familiar with them. All of the pupils could have heard all of the letters named by the one who was being tested, but as their attention was required by the recitation, few of them could follow the pupils as they named the letters. Even if they had heard every letter named, it could have helped them but little, for the letters were called wrong almost as often as right, and those tested last showed no improvement over those tested first. Some pupils could see the letters so far that it was necessary to place them out in a hall, or in another room, wherever conditions permitted it.
Many pupils educated themselves very rapidly in the recognition of the letters, especially the “illiterate E.” It was not an uncommon thing for a pupil to have to move up to twelve feet in order to make out the direction of the E; but having once clearly seen it, he could recognize the other directions with apparent ease at twice the distance. The results of this test are presented in the table on page 179, which gives the number of eyes examined in grades I to VIII, with the distances at which the alphabet and illiterate characters could be distinguished. Four hundred and seventy pupils, or 940 eyes, were examined. These pupils were distributed throughout the grades as follows: 72 in the first grade, 54 in the second, 39 in the third, 28 in the fourth, 41 in the fifth, 29 in the sixth, 130 in the seventh, and 77 in the eighth. Of these pupils, 344?those in grades III to VIII, inclusive? were tested with both the alphabet and illiterate characters. In the column headed, Totals III to VIII, may be found the figures affording the most ready comparison of the results with the alphabet and illiterate cards. Thus, with the alphabet characters, the largest number of eyes, 148, distinguished the letters at fourteen feet; the next largest number, 128, at sixteen feet; and the next largest, 114, at twelve feet. Of the G88 eyes tested, 390, or 5G.7 per cent, distinguished the characters at from twelvo to sixteen feet. With the illiterate cards, however, these same eyes distinguished the character at a much greater distance, 94, the largest number, at twenty-two feet, 92 at twenty-four feet, and S6 at twenty feet. But a smaller number of eyes, only 272, or 39.5 per cent, distinguished the character at these most frequent distances. The results of this comparative test with the two characters are exhibited in figure 1 on page 181, by Curve I for the alphabet test and by Curve II for the illiterate test. The pupils of the first and second grades, 126 in all, were tested with the iliterate cards only, because they did not all know the alphabet. Curve III represents the results obtained by testing these 252 eyes with the illiterate cards. Curve IV combines in a single curve the results of the illiterate test with the 470 pupils in all the grades.
The distances in feet at which the eyes of 470 pupils of the Centennial Grammar School, Trenton, N. J., distinguished the Snellen Alphabet and Illiterate Cards. Feet Grade VIII. Alph. Illit Grade VII. Alph. Illit Grade VI. Alph. Illit Grade V. Alph. Illit Grade IV. Alph. Illit Grade III. Alph. Illit Totals III.-VIII. Alph. Illit Gr. II. Illit. Gr. I. Illit. Totals I and II Illit. 0 1 2 3 4 5 6 8 10 12 14 10 18 20 22 24 26 28 30 32 34 36 38 40 42 44 Tota 1 No. of Eyes 1 1 6 2 0 4 5 5 26 43 34 14 3 4 6 0 0 0 0 0 0 0 0 0 0 154 0 0 1 1 0 5 6 1 2 2 3 17 25 25 28 18 6 1 4 0 0 4 0 4 7 20 39 46 54 45 23 12 4 2 0 0 0 0 3 0 6 0 0 0 0 0 0 0 0 0 154 260 0 0 0 1 0 3 4 2 10 21 13 24 40 41 38 21 24 8 6 4 0 0 0 0 0 260 1 0 0 3 0 2 1 2 6 12 15 8 3 5 0 0 0 0 0 0 0 0 0 0 0 58 0 0 1 0 0 1 0 3 1 0 2 1 4 1 4 8 2 7 9 4 2 4 2 0 2 58 0 1 1 1 1 5 0 7 12 22 23 6 3 0 0 0 0 0 0 0 0 0 0 0 0 82 0 0 0 0 1 0 1 1 4 0 3 4 1 8 4 5 18 20 6 5 1 0 0 0 0 82 0 0 0 0 0 1 9 11 7 10 10 6 2 0 0 0 0 0 0 0 0 0 0 0 0 56 0 0 0 0 0 0 1 1 0 2 2 9 7 9 5 1 7 5 3 4 0 0 0 0 0 56 4 1 3 0 2 7 7 23 17 7 1 6 0 0 0 0 0 0 0 0 0 0 0 0 0 78 1 4 0 0 2 1 1 4 1 5 4 5 9 10 13 5 6 6 1 0 0 0 0 0 0 78 6 3 14 6 7 26 42 87 114 148 ? 128 ? 63 | 23 g 13? 8 0 0 0 0 0 0 0 0 0 0 688 1 4 2 2 3 10 13 12 18 30 ? 27” 60 g 86 g 94 o 92 58 63 47 29 20 9 4 2 0 2 0 0 0 2 0 3 3 4 3 6 11 5 7 10 18 24 6 4 2 0 0 0 0 0 0 108 4 2 0 3 0 1 6 4 3 3 7 16 8 11 38 18 15 5 0 0 0 0 0 0 0 144 4 2 0 5 0 4 9 8 6 9 hh 18 H 21 g 15 ? 21 o 56 42 21 9 2 0 0 0 0 0 0 252
Assuming, in accordance with the principles of Snellen’s construction, that the characters could be read by the normal eye at sixteen feet, we are obliged to conclude that only those eyes are normal that read the characters at just sixteen feet, and that all those eyes are subnormal that could read them at a greater or less distance.
The straight line a-b running perpendicularly through the curves is the line of assumed normal vision for all of the curves. If this were correct, then all of the eyes represented by the portion of the curves to the left of this line are subnormal, being near sighted, while those represented by the portion of the curves to the right of this line are likewise subnormal, being far sighted. Tims we obtain the following results:
Eyes normal. Eyes near sighted. Eyes far sighted. Curve I (alphabet test)… 128 or 18.G% 453 or 65.8% 107 or 15.5% Curve II (illiterate E)… 27 or 3.9% 50 or 8.1% 5GG or 82.2% Taking into consideration the manner in which these tests were made, it is evident at a glance that something is wrong, either with the letters used or with the “illiterate E.” It is impossible for both to be correct, because with the “illiterate E” 8.1 per cent of the pupils tested were near sighted and 82.2 per cent were far sighted; whereas with the alphabet test G5.8 per cent of the same pupils were found to be near sighted and only 15.5 per cent far sighted. The fact that only 3.9 per cent of the pupils, according to the illiterate test, had noijmal vision, and only 8.1 per cent were near sighted, would warrant the suspicion that something was wrong with the “illiterate E.” A close analysis of this letter will, I think, prove that the suspicion is well founded. The structure of the E is shown at the right-hand side of figure 1.
A pupil being tested with this character has only to determine in which one of four directions,?up, down, right, or left,?the opening of the character is directed, and this opening can always be pointed out by observing that this side is the lightest. Each of these characters is constructed in a square which is subdivided into twenty-five small squares. The normal eye is supposed to be able to discern one of these small squares at the same distance that is allowed for recognizing the direction of the character. It will be observed that there are three of these small squares lying together unfilled on the open side of this character. Four of these squares should bo discerned at thirty-two feet, if they were arranged in the form of a square. It would therefore be reasonable to suppose that these three squares should be seen at about three-quarters of that distance, or twenty-four feet. As a matter of fact, this is exactly the distance at which the largest number of the pupils of all grades (Curve IV), did discern them. I therefore feel justified in saying that the distance at which the normal eye should discern the character E should be twenty-four feet instead of sixteen feet.
It may be theoretically correct to say that all eyes that could not see the letters on the sixteen foot line were subnormal,? near sighted,?and that all eyes that could see the letters further than sixteen feet were also subnormal,?far sighted,?but this does not mean that all such eyes need the attention of an oculist. I think, however, it is safe to say that all eyes with T*V vision, or less, and all eyes with f i vision or more, need such attention. Taking these numbers as representing the least degree of deficiency requiring attention, it appears from the table that out of 688 eyes tested, 15.1 per cent, or 104 eyes, may be classed as sufficiently near sighted to need attention, and 1.2 per cent, or 8 eyes, are sufficiently far sighted to need attention.
8 10 IZ 14 !6 Id 20 14 Z6 lQ 30 31 31 36 33 40 4244 182 THE PSYCHOLOGICAL CLINIC. The results for each grade are as follows: Grade. Eyes tested. Eyes &. or below. Eyes ?% or above. 3 78 24 or 30 8% 0 or 0 % 4 56 10 or 14.3% 0 or 0 % 5 82 9 or 11 0% 0 or 0 % 6 58 7 or 12.1% 0 or 0 % 7 260 35 or 13 5% 2 or 0 8% 8 154 19 or 123% 6 or 3.9% 688 104 15 1% 8 1.2%
The first and second grades were not tested with the alphabet, but with the “illiterate E” only, as I have said. Assuming that the normal distance for distinguishing this character is twentyfour feet, instead of sixteen feet, we have 56 normal eyes, 76 far sighted, and 120 near sighted. Again assuming, as in the alphabet test, that those eyes which could see the characters at only half the normal distance, twelve feet, or less, need attention, and that those eyes which could see the characters at thirty-six feet or over also need attention, we find that there are 38 eyes, or 15.1 per cent, so near sighted, and none so far sighted, as to need attention.
My experiment proves conclusively that sixteen feet is not the distance at which the normal eye can just discern the “illiterate E.” In fact, I found twenty-nine eyes, exhibiting normal vision with the alphabet test, that could interpret these characters at twice sixteen feet, and two of this number could do so at forty-four feet. If this character is used, then a new distance for the normal eye must be determined.
If one finds such variations in test types that are supposed to be accurately constructed, one may, with good reason, conclude that the types and characters prepared by many optical concerns with no guidance as to the principle upon which the letters are constructed, are even more likely to give unreliable results; and if there are such differences in the characters used in testing the eyes of school children in different parts of the country, the published result-s must be of but little value for comparison. In an affair of such vital importance to the nation as the testing of its children’s eye-sight, there surely ought to be a uniform standard of measurement. It would be well if the Bureau of Education at Washington would take the matter under consideration, and establish a standard that could be registered and protected as are the weights and measures used in commercial transactions.
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