To determine whether or not this form of exercise is beneficial to the patient, the weekly cycloduction test, as previously described, should be made and compared with the original findings. [Illustration: _A Compact Phorometer and Trial-Frame._ SKI-OPTOMETER MODEL 235 For Testing and Correcting Muscular Imbalance—Providing a Comfortable Form of Trial-Frame.] CHAPTER XV MOVEMENTS OF THE EYEBALLS AND THEIR ANOMALIES After a careful study of the foregoing chapters, the refractionist may desire _further_ knowledge concerning muscular imbalance—a matter in which the Ski-optometer plays an exceptionally important part. It should be remembered that it is only the general utility of the instrument, _plus_ one’s knowledge of refraction and individual diagnosis that enables the refractionist to attain maximum efficiency in every examination, a fact which largely accounts for the following chapter. MONOCULAR FIXATION When we view an object _directly_, so that it appears to be more distinct than surrounding objects, we are said to “fix” or “fixate” it. As the fovea is normally the most sensitive part of the retina, affording by far the most distinct vision, “fixation,” in the great majority of cases, is so performed that the image of the object that is “fixated” falls upon the fovea of the eye that is “fixing.” This is known as central or muscular “fixation.” When central vision is absent, however, the patient is compelled to see with a portion of the retina _outside_ of the fovea. The eye must then be so directed as to cause the image of the object to fall on this outlying portion of the retina. This is termed “eccentric fixation,” and usually denotes that vision is exceptionally poor. The ability to “fix” is apparently acquired in early infancy by constant practice in looking at objects. Any marked interference with vision, particularly with central vision—present at birth or soon thereafter—will tend to prevent the acquisition of this ability, and in _extreme_ cases the eye does not learn to “fix” at all, but aimlessly wanders in all directions. BINOCULAR FIXATION We habitually use the eyes together, fixating with both at once; that is, we direct the eyes in such a way that the image of the object to which the attention is directed falls on the fovea of each eye. Where both eyes are accurately directed to an object at which one or both are looking, the condition is known as “binocular fixation,” which is commonly understood to mean that _both eyes are straight_. The ability to produce and maintain binocular fixation—to keep both eyes directly straight—is acquired early in life. The impulse to maintain it grows with exercise, and soon becomes so strong that after the age of infancy binocular fixation is present in the great majority of persons, and in most of them is present all the time. Binocular fixation must be distinguished through three conditions—_orthophoria_, _heterophoria_ and _squint_. ORTHOPHORIA