and very unwomanly, and in spite of all that has taken place, you are not justified in such revenge.”
“Revenge! How little do you know of me. But I must not talk with you upon this. Even now, while you are almost hating me in your heart, you need my help. What do you intend to do with Henderson? She is not your sister—she has no pity for your weakness and folly, and she does not recollect being one of two children walking about a great garden with their arms round one another’s neck, and talking innocently of love and marriage, but vowing to one another never to be separated—she is enraged with you, and at all events she has her own battle to fight.”
OCULAR STEREOSCOPY.
Although we are all familiar with that ingenious and beautiful instrument, the stereoscope, I think there would be found upon inquiry but a small number among the readers of this article who are equally well acquainted with the fact that ninety-nine persons out of every hundred are provided with a perfect substitute for this apparatus in their own unaided eyes. That the eyes are capable, without external assistance, to produce all the phenomena of stereoscopic vision is a fact, but the comparative rarity with which a natural aptitude for using them stereoscopically occurs may account for the general ignorance in the matter.
There are a few persons, however, in whom, from some cause or other, there exists a natural capacity for ready convergence of the optic axes (in the manner hereafter described) upon any required point. Possessing this power myself, and consequently dispensing with the assistance of the stereoscope, I have been so frequently asked by friends, anxious to do the same, to say how it may be managed, and describe, if possible, some simple means by which the art may be acquired, that I have given the subject a little consideration, and propose to beg the attention of a larger audience for a short explanation of the method by which this end may be accomplished. Previously, however, to proceeding with details, it will be necessary that we should all know a little something of the principles on which the phenomena of stereoscopic, or more properly binocular, vision depend. This subject has been treated so much at length by several modern writers, that I shall do no more than give, in a rapid and very much simplified form, the results of their researches and experiments.
Binocular vision, then, or the seeing with two eyes, is a most important element in the faculty of sight. To this we owe all our real sense of distance and relief; with one eye alone we could have no further conception of solidity than such as is furnished by our judgment and experience; the single eye would see only a flat landscape or unrelieved statue where this strange “second sight” shows us substantial realities, possessing all the charms of nearness, distance, and relief. This phenomenon depends upon a fact which I will endeavour to illustrate. Everybody is aware that we see two totally distinct and different pictures of any given object with the right and with the left eyes. To prove this, hold up a thin book with its back facing you, when the right eye will see the back and right side, while the left eye sees the back and left side of the volume: thus there are manifestly two pictures of the object completely unlike, painted one on either retina, but forming only a single impression in the brain. A more curious illustration is this. Stand at arm’s length before the looking-glass, shut the right eye, and with a forefinger cover its reflected image; now open the right and shut the left eye, and the finger will follow the change; passing apparently across the nose, and settling with its point upon the left eye. Here again is a marked discrepancy between the pictures presented to either eye. The reasons of this difference are simple enough. Our eyes are fixed at a distance from each other averaging 2½ inches; and since all rays of light travel in straight lines, the angle made by those pencils proceeding from any object we may view, and entering the one eye, differs from that formed by the rays which reach the other in a greater or less degree directly proportionate to the distance of the thing looked at.
The following diagram makes this immediately apparent:
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Here A A represents the eyes 2½ inches apart, and B an object about 12 inches distant from them; the lines C D, E F, and G H, I K, will accurately represent the respective pictures presented to either eye.
Now, this is the process which takes place whatever we may look at, and it is in virtue of this peculiarity that the effects of solidity and relief are produced.
The two pictures thus formed on their respective retinæ, both in themselves flat, combine in the brain into one, the resultant image being characterised by the most perfect sensation of relief.
To many it may seem rather paradoxical to declare that monocular vision is destitute of any real sense of distance; and some, I dare say, will be inclined to deny the fact on the supposed evidence of their own senses. Habit, judgment, and experience are so intertwined with our sense of sight, as with every other human faculty, that it is very difficult at first to realise the truth that one eye does not give the same effect of solidity as two. The following experiment, however, will perhaps help objectors to a solution of the question.
Place upon the table an empty small-mouthed phial, and taking another similar bottle full of water in one hand, shut either eye and approach the phial upon the table; then, without any searching motion stretch your arm quickly out and pour the water from the full bottle fairly into the other. In doing this, although you may not be absolutely unsuccessful, you will not fail to be conscious of a difficulty in judging dis-