Page:Handbook of Ophthalmology (3rd edition).djvu/166

160 covery of the instrument, he made the remark that for an orthopedic treatment of squint, no plan promised more than that of methodical practice with the stereoscope. Javal arrived later at the same opinion.

The stereoscope presents the advantage that each eye has its own separate field of vision, into which suitable objects may be introduced. The only difficulty is, that in many cases the patient is not able to see in the stereoscope the two fields of vision at the same time, but always perceives only that of the eye used for fixation. This may be regarded as a "suppression" of the retinal image in the squinting eye; but it is remarkable that the retinal image of the normal eye can be suppressed with the same readiness so soon as the generally squinting eye is used for fixation. But another interpretation of this phenomenon is possible. One who is generally accustomed to binocular fixation will endeavor to fix binocularly in the stereoscope, but the squinting patient, accustomed to monocular fixation, will endeavor to fix monocularly in the stereoscope. The entire retina of the squinting eye serves in this case only for eccentric vision, and just such stereoscopic experiments are well calculated to demonstrate how difficult it is to perceive distinctly eccentric retinal images even with the normal visual act.

In other cases, after some effort, the patient succeeds in seeing the two stereoscopic fields of vision simultaneously; when this is accomplished, he may proceed to practice in uniting the two retinal images. For this purpose, Javal proposes to place in each of the two stereoscopic fields of vision, a wafer contrasting in color with the background. The distance between the two wafers corresponds to the distance between the lines of vision where they pierce the fields. In order to know from the statements of the patient whether binocular union or only monocular vision takes place, a wafer of another color is placed above one of the first wafers and another below the other. In the binocular image there must appear to be three wafers one above the other.

One must be very careful in drawing conclusions from the statements of the patients. Even when the three wafers are seen to stand perpendicularly over each other, there need be no binoc-