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fitted into Maddox' device, bases in, and then rotating them in opposite directions. Wells offers a very handy pocket phorometer for clinical and bedside use (Fig. 46). To produce more delicate results in the lower prismatic powers, Jackson employs in his rotary prism three prisms, one 15 degree stationary prism and two rotating ones of 7 1/2 degrees each.
MEASURE OF DUCTION POWERS.
Of the four principal prism rotations, above mentioned three of them—namely, abduction, supraduction, and infraduction— are fairly constant in quantity, and authorities agree more or less in fixing the power of:
Abduction from 6 to 8 degrees.
Supraduction from 2 to 3 degrees.
Infraduction from 2 to 3 degrees.1
Much depends on the method used in the determination. The above figures are those furnished by single or separate prisms held before the eye one after another. Rotary prisms, especially as employed by Maddox, or any similar apparatus of graduated increment, indicate somewhat higher average powers of rotation, viz.:
Abduction 8 to 10 degrees.
Supraduction 3 to 4 degrees.
Infraduction 3 to 4 degrees.
The fact seems to stand that if, at the first trial of abduction, it is found to equal 7 degrees, there will be little if any variation in its degree, no matter how often it is tried on the same day or succeeding days, provided only that the same method is used each time. The same relative constancy is found in supraduction and infraduction. Not so, however, with adduction, which by its variation from day to day, and even from hour to hour in some cases, has thrown much confusion into the subject of muscular anomalies. Perhaps this confusion hinges on the neglect of many observers to distinguish between primary adduction (or the prism convergence manifested at the first trial) and cramped or trained adduction (the prism convergence possible when the patient has learned to temporarily dissociate accommodation and convergence). Moreover, personal equation counts for no little variation in the results of different investigators. Stevens1 says: "that although an exact standard of adduction is not to be expected, should the adducting ability fail to reach 50 degrees (prism) after a reasonable amount of practice, it is likely to be deficient." Risley 2 foundthe average adduction for 20 feet in 25 non-asthenopic individuals to be 25 degrees; while Bannister,3 who conducted a series of careful studies on 100 soldiers (all in fine physical condition), found the average adduction for 20 feet to be 14 degrees.
1 These figures hold for 20 ft. or more only. For any distance less than 20 ft. abduction would be greater and adduction less; for instance, at 15 ft. abduction equals about 10 to 12 degrees, and if the eyes are steadily approached, a point will finally be reached where abduction and adduction are equal.
It seems to the authors that Bannister's statistics indicate plainly the average degree of primary adduction, while Stevens and Risley's figures have reference more to trained adduction. However, it must be borne in mind that convergence and divergence are acts so intimately bound up that it is almost impossible to dissociate them, so that the study of one side of the phenomenon necessarily involves the other. For instance, in overcoming prisms bases in (or fusing lights through prisms held bases in before the eyes), a stimulus to divergence is created. The eye before which the prism is placed is compelled to diverge, in order that a single image of the light may be preserved. It will be noticed that we speak here of divergence and not of the external rectus, because all muscles which help to turn the eye out are really included in the test. When the prism is placed before one eye only, all of the diverging is done by that eye, and the other unwaveringly maintains its direction toward the object focused. Yet it does not follow that the testing is confined to the eye that turns, for the turning eye is held in its divergent position and the other in its straight position by a contraction of all the external muscles, and therefore we are, in a certain sense, testing convergence and divergence at the same time. The completed action is really a very complicated one. For example, a 6 degree prism is held base in before the right eye; in order to overcome the momentary diplopia induced by the prism, the abductors of the right eye are called into play and that eye diverged, as can be seen through the prism. Strangely enough, the left eye is held straight, notwithstanding the general rule that movement of one eye in any direction is always accompanied by a similar associated movement of the other eye,1 and that therefore the adductors of the left eye tend to turn that eye in the same direction as its fellow. To neutralize this latter impulse the abductors of the left eye are called upon and their action necessitates, in turn, action on the part of the adductors of the right eye, which would immediately bring the right eye back to parallelism were not the abductors fully occupied in keeping it in such position as to avoid diplopia. Hence the introduction of the 6 degree prism disturbed the divergence primarily and convergence secondarily. The same phenomenon occurs when prisms are placed bases out before one or both eyes. In testing supra- or infraduction a similarly complicated act is excited. Thus: when a 2 degree prism is placed base down before the right eye, the image falls on the right retina below the fovea, in consequence of which the elevators contract and the right eye rolls up, turning its fovea until it reaches the position of the displaced image. Naturally an equal stimulation is sent to the elevators of the left eye (according to the above-mentioned rule for associated eye-movements), but a change in the position of the left fovea would destroy the test so that the upward impulse of the left eye must be met by an equal downward impulse of the right eye, which is prohibitive because of the diplopia that would be sure to follow. Hence, while the elevators and depressors are in equilibrium, they are only artificially and temporarily so. We are in a sense determining the power of elevation as compared with that of depression, but we have been really investigating the limits of equilibrium by prism stimulation of allied and of antagonistic muscles.
1 Norris & Oliver, "System of Diseases of the Eye," Vol. II, 1898.
2 Univ. Med. Mag., January, 1895. 'Annals of Ophthal., January, 1898.
1 Movements induced by prisms not too strong to permit fusion constitute the sole apparent exception to this rule.
Further knowledge of the relations of convergence and divergence may be gained from the use of Landolt's ophthalmodynamometer (Fig. 47). Just as in the study of the accommoda
Fig. 47.—Landolt's ophthalmodynamometer.
tion, we learn its amplitude by estimating the position of the punctum proximum and the punctum remotum, so in the study of convergence we find that its amplitude is equal to the difference between the maximum and the minimum of convergence. Landolt's device consists of a blackened metallic cylinder which is to be fitted on to a candle. The side of the cylinder turned toward the eye presents a narrow slit (0.3 mm.), which is illuminated by the flame of the candle and serves as a fixation object. Beneath is attached one end of a spring tape-measure, graduated on one side in centimeters and on the other in the corresponding numbers of meter-angles (or what amounts to the same thing, in diopters). To find the punctum proximum of convergence, the case holding the tape-measure is held beside one of the eyes and the cylinder drawn about two-thirds of a meter away from it, directly in front of both eyes, with the illuminated slit turned toward the patient, who then fixes on the light streak while it is moved directly in the median line closer and closer to the eyes. The moment the streak of light begins to broaden or double, the point of greatest possible accurate convergence has been reached, when one side of the tape will indicate in centimeters the distance of the punctum proximum of convergence, and the other side the corresponding maximum of convergence in meter-angles; for instance, a convergence nearpoint of u c.c. corresponds to 9 meter-angles. In the mentally dull, the examination can be made much easier by having a colored glass before one eye so that doubling of the light streak will be recognized the instant it occurs. A much simpler method that we have used for 6 to 8 years past, is to employ the small electric light used in making the Maddox rod test at the reading distance. If in a darkened room this small light is held 13 inches from the eyes of the patient a tiny bright image of it is seen in the center of each cornea. As the light is carried toward the eyes and they follow it in, the images also move inward toward the nose; but when the light has been approximated to within about 4 inches of the root of the nose, one or both eyes of the average patient will refuse to further converge and the bright corneal image will be seen to roll outward. The moment the break occurs marks the near point of convergence. For instance, if one eye breaks away from the convergence act at 5 inches this is the convergence near point (C. N. P.) and expressed in meter angles would equal 8 meter angles. A 3-inch convergence near point would equal 13 meter angles (M. A.), and a 2-inch convergence near point would equal 20 meter angles of convergence.