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152 THE ADVANCE OF PHOTOGRAPHY
If in the fig. (69) we consider the points a and a' to represent the same point in the view on the stereoscopic pictures obtained, and L L the two lenses of the viewing apparatus, the blending of the images to form one can be understood.
The two corresponding points a a', which belong to the right and left images, appear therefore in common to both eyes at a" — that is, at the same place — and consequently our two eyes see only one image instead of two.
Now, every one who wishes to see an object (for instance, writing) clearly and distinctly, holds it at a definite distance from his eye. This distance is the distance of clear vision. In the case of good eyesight it is eight inches ; with far-sighted persons it is more, and with short-sighted persons less. In the case of stereoscopic vision, the image L» appears remoter or nearer, according as it is removed from or approached towards the two lenses. If the image is near the lenses, it appears when viewed through them nearer and smaller. In the opposite case it appears farther and larger. But every one wishes to see the image at the distance of clear vision, therefore stereoscopes must have movable lenses, in order that persons may adapt the position of the object to the eye — that is, that they may vary the distance of object and lens until the image appears clearest. If such focussing arrangements are absent, the instrument is only adapted for eyes of average power of vision, and requires an effort in eyes of a different calibre. Persons are often met with whose eyes are not of equal strength, one being short and the other long-sighted. There can be no satisfactory stereoscope for such persons : for if the distance of the lenses is adapted for one eye, it does not suit the other.
Nevertheless, such persons can obtain a tolerable