These designate the principal meridians of the astigmatism. Or the stenopaic slit may be rotated in front of the eye until a point is found where the vision is most distinct, which will designate one of the principal meridians, and as the emeridians are always at right angles to one another the other meridian is determined at the same time. Convex and concave glasses are now placed in front of the slit and the degree of hyperopia or myopia, if either exists, ascertained. Next, rotating the slit to a position at right angles to the first the same procedure is again followed out. If convex lenses improve or do not make the vision worse in one meridian, and concave lenses fail to improve it in the other, the case is one of simple hyperopic astigmatism. If concave glasses improve the vision in one position, and convex glasses make it worse in the other, simple myopic astigmatism is present.
If convex glasses improve or do not make vision worse in both positions, it is a case of compound hyperopic astigmatism. The difference between the strongest convex glass in each position represents the astigmatism, and the weaker of the two, thus found, the hyperopia. Compound myopic astigmatism is determined in the same manner by the difference between the two weakest concave glasses.
If the case is mixed astigmatism, convex glasses will improve or will not make the vision worse in one position and concave glasses will improve the vision in the other.
Numerous combinations and variations of these methods are made by different surgeons, but the same principles hold good throughout.
After the astigmatism is determined by any of these methods, it is usual to place the correcting lenses in the frames and have the patient look at the clock face, when, if the astigmatism is properly corrected, the lines will all appear similar.
In all cases of astigmatism, or in any case where spasm of the accommodation is found or suspected, the test should also be made under the influence of a cycloplegic.
Cycloplegics.-By Cycloplegics are meant those drugs which produce temporary paralysis of the ciliary muscles, and therefore suspension of the accommodation. The importance of this in determining ametropia has been stated in the preceding chapter. In addition, however, complete physiological rest of the eyes is obtained which often removes congestive conditions of the retina and choroid, and later when glasses are prescribed they give more comfort than they would have done without the use of a cycloplegic. The drugs most commonly employed are the sulphates of atropine, hyoscyamine and duboisine and the hydrobromates of homatropine and scopolamine.
(1) Atropine is usually employed in a strength of four grains to the ounce. Ordinarily it paralyzes the accommodation in about two hours and the effects remain for a week. A drop of the solution should be dropped into the outer canthus three times during one day. In cases of marked spasm of the accommodation in young hyperopic subjects it can be continued for several days.
(2) Hyoscyamine and Duboisine are employed in the form of solutions made up of two grains to the ounce. Their action is much more rapid than atropine and their cycloplegic effect more transitory.
(3) Scopolamine may either be employed in a one per cent. solution, a single drop being instilled, or in a one-fifth per cent solution, one drop every fifteen minutes for an hour and a half. Cycloplegia occurs in about forty-five minutes and lasts from three to five days. Toxic symptoms sometimes develop, so considerable care should be exercised.
(4) Homatropine used in a three per cent. solution, one drop being instilled every fifteen minutes for an hour and a-half preceding the examination, can be employed when a very transitory effect on the ciliary muscle is desired. Its effect is increased by dropping a drop of a four per cent. solution of cocaine in the eye each time before instilling the homatropine. The cycloplegic effects of homatropine pass off in about fifty hours, and are in a degree neutralized by eserine.
It is not safe to use strong cycloplegics in elderly people on account of the danger of precipitating an attack of glaucoma. Of course, they must never be employed if glaucoma is suspected or present. It is unnecessary to use them in people whose advanced age denotes that the accommodative power is very weak.
Patients soon become familiar with the letters on a test card, and children are apt to memorize them before being tested, so it is advisable to have several cards with different letters. Thus a new card should always be displayed for each eye, and if at any time there is any suspicion that the patient is drawing upon his memory a different one must be substituted. In order to avoid the necessity of walking across the room each time to make such a change, and especially in order to be able to make it without the patient’s knowledge, I devised a changeable test- type, the plans of which were presented to Messrs. Clairmont & Co., of New York, who made the apparatus and perfected the motor for working it. This instrument was described in a paper read before the Homoeopathic Medical Society of the County of New York in 1893.
It is illustrated in Fig. 22.
The apparatus consists of an ornamented wooden case, upon which are mounted the ordinary Snellen test-types. The five lower lines only are capable of being changed, the changes being produced by the revolution of five quadrangular rollers permitting the exhibition of four series of letters. Motor power is furnished by an accordion-pleated rubber tuber, which, when expanded by the column of air communicated to it by the pressure of a bulb, elevates a weight to which is attached an arm. As the arm moves upward it carries a cog which locks with the wheel that revolves the five rollers. This wheel contains four slots, placed at intervals of ninety degrees, for the reception of two catches, an upper and a lower one, which limit the revolution of the wheel to a quarter of a circle. After each quarter revolution the weight carries the arm back to its former position, setting the apparatus for the next change. It is operated by a bulb at the side of the patient, which is connected with the motor by tubing.
An instrument called the Refractometer has been invented by H.L. De Zeng, whose purpose is the estimation of the total refractive error and particularly the whole amount of astigmatism present in all the dioptric media without the use of a cycloplegic.
This instrument shown in Fig. 23 is manufactured by the Cataract Optical Co., of Buffalo. While disclaiming that any mechanical device can wholly replace the ordinary test under a cycloplegic this instrument is certainly of great value where a cycloplegic is contra-indicated or refused, as well as in general routine work.
In brief this instrument consists of a nickel tube, in the head of which is placed a stationery concave lens of 20. D. It also contains an inner tube which is movable along its cylindrical axis by means of a rack and pinion adjustment, and which carries at its front end a convex achromatic objective. These lenses in combination at different distances give all the spherical foci from plus.12 D to plus 18. D, and from -.1 D to – 0. D inclusive. The convex spherical effects are recorded upon a revolving dial at the side and the concave effects upon the top of the inner tube, visible to the observer through an oval opening in the top of the outer tube.
Owing to the range of the negative scale being limited to-9. D, two auxillary caps accompany the instrument, one containing 10. D and the other-20. D, which, when placed over the eye piece raises the negative scale to either-19. D or-29. D respectively. The outer tube is further armed at its front end with a revolving head, composed of two revolving discs, containing blanks, a stenopaic slit, and eleven minus concave cylinders set at right angles with their radii. The resulting combinations possible give a range of cylindrical lenses from-.12 D to-8.75 D, which can be rotated to any given axis, the latter being indicated.
By reason of the instrument’s optical construction, it has an amplification of two and one-third diameters, and in consequence of this the test-types furnished with it are reduced to three-sevenths of the size of Snellen’s letters, so that the visual acuity may be reliably estimated with the instrument. The instrument must be properly adjusted for whatever range is desired, either 3,4,5 or 6 metres.
The best method employed in testing is what is known as the “fogging system,” which consists in over-correcting a hyperopic eye with convex lenses or under-correcting a myopic one with concave lenses which are too weak. The effects of this is to render the lines and letters deeply blurred, which causes relaxation of the ciliary muscle and in consequence latent errors to become manifest. With the instrument properly adjusted, and the patient properly placed, the thumb-screw is turned until the test-letters are distinctly seen and the reading noted.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.