TECHNICAL
NOTE
CONTACT LENSES FOR ANIMALS USED IN VISION RESEARCH’ CHRIS-ITAN GULD and
ALGISBERTULIS’
Institute of Neurophysiology, University of Copenhagen. Denmark
(Rrcrircd 6 June 1974)
In investigations of vision where it is necessary to keep the eye open for many hours it is customary to use a contact lens to prevent the cornea from drying. The problem is to fit the inner radius of the contact lens to that of the cornea. Wiesel and Hube! (1966) used a keratometer to measure the cornea! radius. whereas other workers chose a suitable lens from a bank of lenses and corrected the eye to neutrality with correcting lenses in front of the eye. We are describing a method to determine the cornea! radius by retinoscopy and with contact lenses of known inner radius. The method and its theoretical basis are similar to those used in humans when ocular refraction was corrected with a glass contact lens with a thin fluid lens between it and cornea (Duke-Elder. 1970). The animals (vervet monkeys. Cercopithecus aethiops) were anesthetized with Ketalar@ and the pupil was dilated with I”/,: homatropine. The ocular refraction d, D (diopter) of the eye was measured by retinoscopy. Then a contact lens with inner radius r. and power d,D was placed on the eye and the refraction d2D of lens and eye was measured. In this case a thin fluid lens of unknown power d, lies between contact lens and cornea. Ocular refraction measured by retinoscopy gives the power of a lens which, when placed near to or on cornea. corrects the eye to neutrality. Thus a lens of power d,D corrects the eye, as does the summated power of the contact lens. the fluid lens and a lens of power dzD. For low powers we have therefore the equation d, = d, + d, + d2, from which the power of the fluid lens is d, = d, -
d2 -
d,.
(1)
It is valid to regard the three elements--contact lens. fluid lens and cornea-as separated by two infinitely thin air spaces. Then d, is the power of the fluid lens ’ Supported by grants from the Danish Medical Research Council. 2 Dr. Bertulis was on leave of absence from Department of Biology. Kaunas Medical Institute, Kaunas Lithuania. as exchange fellow under the agreement for cultural and scientific exchange between the U.S.S.R.and Denmark. 441
in air, and the relation from thin lens theory is applicable: d, = (n, -
I!(:
-
;\
\ro
‘91
where nI is the refractive index of the fluid. r,, is the anterior and r,, the posterior radius of the fluid lens. r,, is also the inner radius of the contact lens and rp the radius of cornea, derived from equation (2) as r9 =
rs I - dlrJ(nf
-
I)’
(3)
Usually the fluid lens is constituted of tears. which have a refractory index nI = 1.336, and equation (3) is reduced to ru (4) ” = I - 3.0drr,’ The accuracy of retinoscopy is at best about 025 D. With cornea1 radii of S-6 mm in equation (4) the error is about DO2 mm. In our measurements we allowed d, to be up to +- 3.0 D. Then. from equation (I) and with rp and r. in mm. the cornea1 radius derived from equation (4) is r,, = r. +
3 x lo-’ (d, - d, - di)r,,‘.
(5)
For each eye, r9 was determined by measuring with at least two contact lenses of different radius. Such measurements could deviate by O-02-005 mm. The contact lens was ground with the inner radius given by the mean of these measurements and to correct the refraction d, of the eye. Provided with these lenses the eyes were neutral to within +_@25D. We used Maxwellian illumination of the retina and saw the spot on retina through a half mirror (Guld and Bertulis in preparation). Illumination of retina was less than that from a screen with a brightness of lo* cd/m’ to prevent long-lasting effects on retina. The faint reflection from the foveola was placed in the middle of a small light spot focused on retina and its position determined with an accuracy of l/lo”. This procedure could be r-ated during experiments lasting up to 12 hr without any dripping of the eye or cleaning
432
Technical Note
of the lens. In a few experiments we used contact lenses which, with a thin fluid lens, could compensate for a refraction of c @5 D. Then the image of retina became dim in a few hours due to blemishes on the cornea. In these cases the radius of the contact lens deviated only 005 mm from the comeal radius. In I5 female vervet monkeys the cornea1 radius varied -from 535 to 6.02 mm and the refractive power of the eyes was less than + I.0 D.
REFERENCES
Duke-Elder S. (1970)Sysremoff_3ppkthalmo&gy.Vol. V: Ophthalmic Optics and Rejaction. Kimpton, London. Guld C. and Bertulis A. Representation of central retina on dorsolateral occipital cortex of vcrwt monkey, Cercopithecwaethiops (in preparation). Wicscl T. N. and Hubel D. H. (1966) Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J. Neurophysiol. 29. 1 I 15-l 156.
NOTE
CONTACT LENSES FOR ANIMALS USED IN VISION RESEARCH’ CHRIS-ITAN GULD and
ALGISBERTULIS’
Institute of Neurophysiology, University of Copenhagen. Denmark
(Rrcrircd 6 June 1974)
In investigations of vision where it is necessary to keep the eye open for many hours it is customary to use a contact lens to prevent the cornea from drying. The problem is to fit the inner radius of the contact lens to that of the cornea. Wiesel and Hube! (1966) used a keratometer to measure the cornea! radius. whereas other workers chose a suitable lens from a bank of lenses and corrected the eye to neutrality with correcting lenses in front of the eye. We are describing a method to determine the cornea! radius by retinoscopy and with contact lenses of known inner radius. The method and its theoretical basis are similar to those used in humans when ocular refraction was corrected with a glass contact lens with a thin fluid lens between it and cornea (Duke-Elder. 1970). The animals (vervet monkeys. Cercopithecus aethiops) were anesthetized with Ketalar@ and the pupil was dilated with I”/,: homatropine. The ocular refraction d, D (diopter) of the eye was measured by retinoscopy. Then a contact lens with inner radius r. and power d,D was placed on the eye and the refraction d2D of lens and eye was measured. In this case a thin fluid lens of unknown power d, lies between contact lens and cornea. Ocular refraction measured by retinoscopy gives the power of a lens which, when placed near to or on cornea. corrects the eye to neutrality. Thus a lens of power d,D corrects the eye, as does the summated power of the contact lens. the fluid lens and a lens of power dzD. For low powers we have therefore the equation d, = d, + d, + d2, from which the power of the fluid lens is d, = d, -
d2 -
d,.
(1)
It is valid to regard the three elements--contact lens. fluid lens and cornea-as separated by two infinitely thin air spaces. Then d, is the power of the fluid lens ’ Supported by grants from the Danish Medical Research Council. 2 Dr. Bertulis was on leave of absence from Department of Biology. Kaunas Medical Institute, Kaunas Lithuania. as exchange fellow under the agreement for cultural and scientific exchange between the U.S.S.R.and Denmark. 441
in air, and the relation from thin lens theory is applicable: d, = (n, -
I!(:
-
;\
\ro
‘91
where nI is the refractive index of the fluid. r,, is the anterior and r,, the posterior radius of the fluid lens. r,, is also the inner radius of the contact lens and rp the radius of cornea, derived from equation (2) as r9 =
rs I - dlrJ(nf
-
I)’
(3)
Usually the fluid lens is constituted of tears. which have a refractory index nI = 1.336, and equation (3) is reduced to ru (4) ” = I - 3.0drr,’ The accuracy of retinoscopy is at best about 025 D. With cornea1 radii of S-6 mm in equation (4) the error is about DO2 mm. In our measurements we allowed d, to be up to +- 3.0 D. Then. from equation (I) and with rp and r. in mm. the cornea1 radius derived from equation (4) is r,, = r. +
3 x lo-’ (d, - d, - di)r,,‘.
(5)
For each eye, r9 was determined by measuring with at least two contact lenses of different radius. Such measurements could deviate by O-02-005 mm. The contact lens was ground with the inner radius given by the mean of these measurements and to correct the refraction d, of the eye. Provided with these lenses the eyes were neutral to within +_@25D. We used Maxwellian illumination of the retina and saw the spot on retina through a half mirror (Guld and Bertulis in preparation). Illumination of retina was less than that from a screen with a brightness of lo* cd/m’ to prevent long-lasting effects on retina. The faint reflection from the foveola was placed in the middle of a small light spot focused on retina and its position determined with an accuracy of l/lo”. This procedure could be r-ated during experiments lasting up to 12 hr without any dripping of the eye or cleaning
432
Technical Note
of the lens. In a few experiments we used contact lenses which, with a thin fluid lens, could compensate for a refraction of c @5 D. Then the image of retina became dim in a few hours due to blemishes on the cornea. In these cases the radius of the contact lens deviated only 005 mm from the comeal radius. In I5 female vervet monkeys the cornea1 radius varied -from 535 to 6.02 mm and the refractive power of the eyes was less than + I.0 D.
REFERENCES
Duke-Elder S. (1970)Sysremoff_3ppkthalmo&gy.Vol. V: Ophthalmic Optics and Rejaction. Kimpton, London. Guld C. and Bertulis A. Representation of central retina on dorsolateral occipital cortex of vcrwt monkey, Cercopithecwaethiops (in preparation). Wicscl T. N. and Hubel D. H. (1966) Spatial and chromatic interactions in the lateral geniculate body of the rhesus monkey. J. Neurophysiol. 29. 1 I 15-l 156.
