- Email: [email protected]
A Pocket Tester*
NOTES, CASES, INSTRUMENTS
860
versed in one hand2 and the condensing lens in the other is a convenient method, but limits the surgeon. With this stand, the scope can be brought into the desired position merely by manip ulation of the sterile handle. Since it re mains in place, areas of interest can be demonstrated to other observers. The ex aminer has a free hand for scierai depres-
sion and movement of the globe. Simple sterilization is a further advantage. Most important, however, is the fact that the ophthalmoscope is readily available through out the surgical procedure. Frequent fundus examinations are made without effort, thus creating more favorable conditions for reattachment of the retina. 14401 Hamlin Street.
REFERENCES
1. Havener, W. H.: Schepens' binocular indirect ophthalmoscope. Am. J. Ophth., 45:915 (June) 1958. 2. Ross, M. G.: Use of the Schepens binocular indirect ophthalmoscope in operations. AMA Arch. Ophth., 60:947-949 (Nov.) 1958.
A P O C K E T TESTER* FOR DETERMINING DISTANCE VISUAL ACUITY SAMUEL W.
COHEN,
M.D.
Brooklyn, New York This is the description of a simple, practi cal, inexpensive and accurate technique for measuring the visual acuity of a patient who is seen at his bedside or at home, or under any circumstance where the availability of a Snellen test chart, or projector, or testing space is impractical or impossible. An ordinary Snellen test chart is reduced in size photographically and in proportion to the distance at which the test is made. For example, if one meter is chosen as the test ing distance, the Snellen chart used for or dinary distance visual acuity measurement is reduced to one sixth of its size by the following calculation: The regular-sized Snellen letters are con structed so that the details of the letters at the corresponding distances form an angle of one minute of arc (minimum separable) or the entire letter forms an angle of five minutes of arc at the nodal point of the eye. By proportional triangles, ABN and ♦From the Division of Ophthalmology, The Jewish Hospital of Brooklyn.
A'B'N, as shown in the diagram (fig. 1), it can be seen that the 20/20 letter (6/6) is approximately nine mm. of length and width: 0.005 mm. 1 X Tan 1' = = = 17.05 mm. 3400 6006 mm. X = 1.76 mm. and the entire letter = 5x 1.76 = 8.80 mm. Now, using a similar diagram and if a lens of +1.0 diopter is placed in front of the eye, as in Figure 1A, with C as its cen ter, it will be seen that similar triangles ABN and A"B"C can be constructed by drawing a line B"C parallel to BN and in tercepting the perpendicular from point A " which is placed at the focal length of the
Fig. 1 (Cohen). Construction of Snellen letters.
Fig. 1A (Cohen). Snellen letter with +1.0D. lens in front of eye.
NOTES; CASES, INSTRUMENTS
lens, namely at one meter from C. There fore, X 1/3400 = and X = 0.29 ; 1000 5x = 1.45 mm. That is, the ray from the tail of the arrow and the ray from the head of the arrow, as they travel through and out of the lens and head toward the eye, form an angle of one minute with each other, exactly as do the rays from arrow AB. Since all rays emanating from B", being on the focal plane of the lens, are refracted parallel to the ray going through the center (that is, B"C), there will ultimately be coincidence with the ray going from B to the nodal point N. The rays A"N and AN are likewise coincident. Hence A " B " will have the same distance spread on the retina as AB. 8-80 The ratio of -—— equals 6.06 or ap1.45 proximately 6 That is the Snellen test chart, if reduced by one sixth and held at one meter distance from the patient, holding a +1.0D. lens in front of his eye, will form the same angular measurements as the regular test chart held at six meters distance, without the lens in front of the eye. By similar calculations, it can be shown that at one-tenth meter, and with a +10D. lens, the chart must be reduced by 1/60, and at 1/100 meter, with a +100D. lens, by 1/600, and so forth, to form the same angular measurements. The proper convex lens likewise elimi-
Fig. 2 (Cohen). Using the tester.
861
Fig. 3 (Cohen). A pocket-size viewer may be used.
nates the factor of accommodation for the emmétrope, by changing the vergence of the rays to parallelism (that is, of 0 vergence). For example, a -f-l.OD. for one meter; + 10D. for one-tenth meter; +100D. for 1/100 meter, and so forth, causes the rays from each point of each letter to assume parallelism, and at the same time the pen cils of rays from each point maintain the same angular relationships at the nodal point as the regular size Snellen test letters placed at optical infinity (that is, 20 ft. = 6 meters). Thus, an emmétrope will read the 20/20 line of the one-sixth reduction at one meter with a +1.0D. lens in front of his eye. In other words, the visual acuity recorded is in every respect equivalent to the distance visual acuity recorded with the regular Snellen test chart, placed at optical infinity, with the factor of accommodation suppressed for the emmétrope. We have put a photograph of the Snellen chart, reduced by one- sixth, on a handy pocket-sized card three inches by four inches. Attached to the border of the card is a string one meter long, at the end of which is a trial frame lens of +1.0 diopter. The patient or doctor holds the lens in front of the patient's eye and the free hand of the doctor holds the card (fig. 2) at one meter
NOTES, CASES, INSTRUMENTS
862
distance (as measured by the length of the string). The patient, with his distance-cor rection spectacles on, is instructed to read the figures as far down as he can. The vis ual acuity measurements with this method have approximated closely the results ob tained with the same patients by the usual Snellen method. The great majority come to within one-half of a line difference. The string with the lens is easily wound around the card and slipped into a wallet or pocket, ready for use at any moment, at any place. If preferred, a commercial pocket-size viewer, with a uniform, constant built-in il lumination can be used (fig. 3). The slide size must be calculated according to the de scription already given. All that is needed to be known is the distance between screen and lens and the dioptric power of the lens (which can be supplied by the manufacturer or measured easily by a ruler and a lensometer). For example, if the viewer comes with a +10D. lens and the distance between lens and screen is 100 mm. (1/10 meter), the slide must be a photograph of the Snel len chart, reduced by 1/60. This viewer and slide can be carried in the pocket for a quick efficient measurement of distance visual acuity under a constant, standard, uniform illumination. 135 Eastern Parkway (38). A MODIFIED ERISOPHAKE* E X T R A C T I O N OF A H Y P E R M A T U R E ( M O R G A G N I ) CATARACT A. HAGEDOORN,
M.D.
AND C. M. J. VELZEBOER,
M.D.
Amsterdam, Holland
In a hypermature cataract it is difficult or impossible to grasp the capsule with a cap* From The University Eye Clinic. Ch. G. van den Bosch, technician, and A. J. J. Lammens, medi cal photographer, were major contributors to this work and we wish to express our gratitude to them. Mr. van den Bosch especially contributed much to the development of the instrument.
Fig. 1 (Hagedoorn and Velzeboer). The new erisophake.
860
versed in one hand2 and the condensing lens in the other is a convenient method, but limits the surgeon. With this stand, the scope can be brought into the desired position merely by manip ulation of the sterile handle. Since it re mains in place, areas of interest can be demonstrated to other observers. The ex aminer has a free hand for scierai depres-
sion and movement of the globe. Simple sterilization is a further advantage. Most important, however, is the fact that the ophthalmoscope is readily available through out the surgical procedure. Frequent fundus examinations are made without effort, thus creating more favorable conditions for reattachment of the retina. 14401 Hamlin Street.
REFERENCES
1. Havener, W. H.: Schepens' binocular indirect ophthalmoscope. Am. J. Ophth., 45:915 (June) 1958. 2. Ross, M. G.: Use of the Schepens binocular indirect ophthalmoscope in operations. AMA Arch. Ophth., 60:947-949 (Nov.) 1958.
A P O C K E T TESTER* FOR DETERMINING DISTANCE VISUAL ACUITY SAMUEL W.
COHEN,
M.D.
Brooklyn, New York This is the description of a simple, practi cal, inexpensive and accurate technique for measuring the visual acuity of a patient who is seen at his bedside or at home, or under any circumstance where the availability of a Snellen test chart, or projector, or testing space is impractical or impossible. An ordinary Snellen test chart is reduced in size photographically and in proportion to the distance at which the test is made. For example, if one meter is chosen as the test ing distance, the Snellen chart used for or dinary distance visual acuity measurement is reduced to one sixth of its size by the following calculation: The regular-sized Snellen letters are con structed so that the details of the letters at the corresponding distances form an angle of one minute of arc (minimum separable) or the entire letter forms an angle of five minutes of arc at the nodal point of the eye. By proportional triangles, ABN and ♦From the Division of Ophthalmology, The Jewish Hospital of Brooklyn.
A'B'N, as shown in the diagram (fig. 1), it can be seen that the 20/20 letter (6/6) is approximately nine mm. of length and width: 0.005 mm. 1 X Tan 1' = = = 17.05 mm. 3400 6006 mm. X = 1.76 mm. and the entire letter = 5x 1.76 = 8.80 mm. Now, using a similar diagram and if a lens of +1.0 diopter is placed in front of the eye, as in Figure 1A, with C as its cen ter, it will be seen that similar triangles ABN and A"B"C can be constructed by drawing a line B"C parallel to BN and in tercepting the perpendicular from point A " which is placed at the focal length of the
Fig. 1 (Cohen). Construction of Snellen letters.
Fig. 1A (Cohen). Snellen letter with +1.0D. lens in front of eye.
NOTES; CASES, INSTRUMENTS
lens, namely at one meter from C. There fore, X 1/3400 = and X = 0.29 ; 1000 5x = 1.45 mm. That is, the ray from the tail of the arrow and the ray from the head of the arrow, as they travel through and out of the lens and head toward the eye, form an angle of one minute with each other, exactly as do the rays from arrow AB. Since all rays emanating from B", being on the focal plane of the lens, are refracted parallel to the ray going through the center (that is, B"C), there will ultimately be coincidence with the ray going from B to the nodal point N. The rays A"N and AN are likewise coincident. Hence A " B " will have the same distance spread on the retina as AB. 8-80 The ratio of -—— equals 6.06 or ap1.45 proximately 6 That is the Snellen test chart, if reduced by one sixth and held at one meter distance from the patient, holding a +1.0D. lens in front of his eye, will form the same angular measurements as the regular test chart held at six meters distance, without the lens in front of the eye. By similar calculations, it can be shown that at one-tenth meter, and with a +10D. lens, the chart must be reduced by 1/60, and at 1/100 meter, with a +100D. lens, by 1/600, and so forth, to form the same angular measurements. The proper convex lens likewise elimi-
Fig. 2 (Cohen). Using the tester.
861
Fig. 3 (Cohen). A pocket-size viewer may be used.
nates the factor of accommodation for the emmétrope, by changing the vergence of the rays to parallelism (that is, of 0 vergence). For example, a -f-l.OD. for one meter; + 10D. for one-tenth meter; +100D. for 1/100 meter, and so forth, causes the rays from each point of each letter to assume parallelism, and at the same time the pen cils of rays from each point maintain the same angular relationships at the nodal point as the regular size Snellen test letters placed at optical infinity (that is, 20 ft. = 6 meters). Thus, an emmétrope will read the 20/20 line of the one-sixth reduction at one meter with a +1.0D. lens in front of his eye. In other words, the visual acuity recorded is in every respect equivalent to the distance visual acuity recorded with the regular Snellen test chart, placed at optical infinity, with the factor of accommodation suppressed for the emmétrope. We have put a photograph of the Snellen chart, reduced by one- sixth, on a handy pocket-sized card three inches by four inches. Attached to the border of the card is a string one meter long, at the end of which is a trial frame lens of +1.0 diopter. The patient or doctor holds the lens in front of the patient's eye and the free hand of the doctor holds the card (fig. 2) at one meter
NOTES, CASES, INSTRUMENTS
862
distance (as measured by the length of the string). The patient, with his distance-cor rection spectacles on, is instructed to read the figures as far down as he can. The vis ual acuity measurements with this method have approximated closely the results ob tained with the same patients by the usual Snellen method. The great majority come to within one-half of a line difference. The string with the lens is easily wound around the card and slipped into a wallet or pocket, ready for use at any moment, at any place. If preferred, a commercial pocket-size viewer, with a uniform, constant built-in il lumination can be used (fig. 3). The slide size must be calculated according to the de scription already given. All that is needed to be known is the distance between screen and lens and the dioptric power of the lens (which can be supplied by the manufacturer or measured easily by a ruler and a lensometer). For example, if the viewer comes with a +10D. lens and the distance between lens and screen is 100 mm. (1/10 meter), the slide must be a photograph of the Snel len chart, reduced by 1/60. This viewer and slide can be carried in the pocket for a quick efficient measurement of distance visual acuity under a constant, standard, uniform illumination. 135 Eastern Parkway (38). A MODIFIED ERISOPHAKE* E X T R A C T I O N OF A H Y P E R M A T U R E ( M O R G A G N I ) CATARACT A. HAGEDOORN,
M.D.
AND C. M. J. VELZEBOER,
M.D.
Amsterdam, Holland
In a hypermature cataract it is difficult or impossible to grasp the capsule with a cap* From The University Eye Clinic. Ch. G. van den Bosch, technician, and A. J. J. Lammens, medi cal photographer, were major contributors to this work and we wish to express our gratitude to them. Mr. van den Bosch especially contributed much to the development of the instrument.
Fig. 1 (Hagedoorn and Velzeboer). The new erisophake.