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Tests for Color-blindness. The phenomena stated in the above table are apparently so conspicuous that it seems almost incredible that persons should be color-blind unknown to themselves and their friends. Yet nothing is more certain than that even intelligent persons may have this defect without being at all aware of it. They use the terms red, green, etc., although the sensations corresponding to these terms are different from those experienced by persons of normal eyes. But how are they to know it? They may make strange and unaccountable mistakes sometimes, but these are attributed to the loose use of color-names. The defect is by no means uncommon, and, what is worthy of note, it is far more common among men than among women. Among men perhaps four to five per cent are more or less color-blind; among women hardly more than one tenth per cent. The importance of testing for this defect in the case of engine-drivers and switchmen of railways, and wheelmen and lookouts of vessels, can not be overestimated. There are many methods of testing, some of them very refined and accurate; but for that very reason unadapted for ordinary use. The simplest, and perhaps one of the best, is that of Holmgren. A box full of skeins of yarn of all colors and shades is placed before the subject, and he is directed, without assistance, to sort and match them. All normal-eyed persons will match them similarly and correctly, but in the case of the decidedly color-blind the most extraordinary matchings occur. For example, bright reds and bright greens and certain shades of gray, or scarlet and certain shades of brown, or splendid purples and certain shades of slateblue, are put together as the same color and shade.
SINGLE AND DOUBLE VISION.
The Two Eyes a Single Instrument. We have thus far treated only of the phenomena of monocular vision; and all that we have said might still apply, almost word for word, if, like the Cyclops Polyphemus, we had but
in the middle of the forehead. But we have two eyes; and these are not to be considered as mere duplicates, so that if we lose one we still have another. On the contrary, the two eyes act together as one instrument; and there are many visual phenomena, and many judgments based upon these phenomena, which result entirely from the use of two eyes as one instrument. These form the subject matter of Binocular Vision. It must be clearly understood that the distinctive phenomena of binocular vision require two eyes acting as one. We might have two eyes, or even, like Argus, a hundred eyes, and yet not enjoy the advantages of binocular vision ; for each eye might see independently. This would still be monocular vision.
The phenomena of binocular vision are far less purely physical than those of monocular vision. They are also far more obscure, illusory, and difficult of analysis, because far more subjective and far more closely allied to psychical phenomena. From early childhood I have amused myself with experiments in this field, and have thus acquired an unusual voluntary power over the movements of the eyes, and a still more unusual
power of analysis of visual phenomena. This has always therefore been a favorite field for me; but with a little practice any one may acquire similar power and enjoy a similar pleasure.
Binocular Field. We have said that the field of view is naught else than an outward projection of retinal states. With the eyes open and the retina in an active or stimulated condition, we call it the field of view; with the eyes shut and the retina in a comparatively passive or unstimulated condition, we call it the field of darkness. In either case, every variation in the state of different parts of the retina, whether by
shadows or by images, or by its own internal changes or unstimulated activity, is faithfully represented in external space by spectra, external images, etc. But we have two eyes, and therefore two retina, and therefore also two fields of view, the external projections of the two retinæ. These two fields of view partly overlap each other, so as to form a common or binocular field. Fig. 35 represents roughly the form of these fields in my own case. The right field, R, is bounded by the line of the nose n n on the left, the brows br above, and the cheek ch below. The field of the left eye, L, is bounded similarly on the right by the nose n' n', the brow br', and the cheek ch'. Between the lines of the nose, n n, n' n', is the rounded triangular
space CF, which is the common or binocular field. This common field is the only part seen by both eyes. The two fields are left vacant on the extreme right and left, because, projected on a plane surface, they are unlimited in these directions. This is the necessary result of the fact that in a horizontal direction the field of view of both eyes is more than 180°.
Now, there being two retinæ, there are of course two retinal images of every external object; and since retinal images are projected outward into space as external images, we must have two external images of every object. But we see objects only by these external images. Why, then, with two retinal images—ay, and two external images for every object, do we not see all objects double ? I answer: We do indeed see all objects double, except under certain conditions.
Double Images This phenomenon of double images of all objects, except under certain special conditions, is so fundamental in binocular vision, and yet so commonly overlooked by even the most intelligent persons unaccustomed to analyze their visual impressions, that it becomes absolutely necessary first of all to prove it by detailing many experiments, which every one may repeat for himself.
Experiment 1.--Holding up the finger before the eyes, look, not at the finger, but at the wall or the ceiling or the sky. Two transparent images of the finger will be seen, the left one belonging to the right eye and the right one to the left eye. We easily prove this by shutting first one and then the other eye, and obserying which image disappears. The images are transparent, or shadowy, because they do not conceal anything. The place covered by the right-eye image is seen by the left eye, and the place covered by the lefteye image is seen by the right eye. If we alternately shut one eye and then the other, the wide difference between these places is at once evident. Usually there is an alternation in the distinctness of these shadowy images—first one and then the other fading away, and almost disappearing from view. Many persons find difficulty in consciously recognizing the two images. Such persons habitually neglect one, until it finally drops out of consciousness—which one they neglect will be shown in the next experiment.
Experiment 2.—Point with the forefinger at some distant object, looking with both eyes open at the object, not the finger. Two fingers will be seen, one of them pointing at the object and the other far out of range, usually to the right.
Most persons find some difficulty at first in being conscious of perceiving two images. The reason is, they do not easily separate what they know from what they see. They know there is but one finger, and therefore they think they see but one. The best plan is to shut alternately one eye and then the other, and observe the places of projection of the finger against the wall; and then, opening both eyes, shadowy images at both these places will be seen. I have found