Popular Scientific Recreations in Natural Philosphy, Astronomy, Geology, Chemistry, etc., etc., etc. Gaston Tissandier

of Contents

      VISION—THE EYE—THE STEREOSCOPE—SPECTRUM ANALYSIS—THE SPECTROSCOPE—THE TELESCOPE AND MICROSCOPE—PHOTOGRAPHY—DISSOLVING VIEWS—LUMINOUS PAINT.

      The eye and vision are such important subjects to all of us that we may be excused for saying something more concerning phenomena connected with them, and the instruments we use for assisting them. We do not propose to write a treatise upon the physiology of vision, for we know the image in the eye is produced physically in the same manner as the image in a camera obscura. In the eye the sides of the box are represented by the sclerotic (see chap. x. fig. 95); the dark inner surface has its parallel in the pigment of the choroid; the opening in the box in the pupil of the eye; the convex lens in the crystalline and the cornea; and the retina receives the image. But why we see—beyond the fact that we do see—no one can explain. Science is dumb on the subject. Thought and consciousness elude our grasp; and, as Professor Tyndall says on this subject, “we stand face to face with the incomprehensible.”

      But there are many interesting facts connected with our vision which may be plainly described. Some people are obliged to carry an object (or a book) to within ten inches of the eye to see it distinctly; and a person who does not possess convergent power of the eye will have to move it farther off, or use convex glasses; while a “near-sighted” person, whose eyes are too quickly convergent, will use concave glasses to bring the object near to the eye.

      There is but one small place in the retina of the eye which admits of perfect vision. This, the most sensitive portion, is called the “yellow spot,” and vision becomes more and more indistinct from this point towards the circumference. This can be proved by any one; for in reading we are obliged to carry our eyes from word to word, and backwards and forwards along the lines of print. Another very important element in our vision is the contraction and enlargement of the iris around the pupil. In cases where strong light would only dazzle us the iris expands, and the pupil is contracted to a sufficient size to accommodate our vision. At night, or in a darkened room, the pupil is enlarged. This change will account for our not being immediately able to see objects when we have passed from darkness to great light, or vice versâ. The iris must have time to accommodate itself to the light.

      Now, outside the small space of perfect vision above mentioned, there is a circle of considerable extent, called the “field of vision.” In man this field, when the eyes are fixed, subtends an angle of about 180°, because beyond that the rays cannot enter the pupil of the eye. But in the lower animals, the fish and birds—notably the ostrich—the field of vision is much more extensive, because the pupils are more prominent, or the eyes are set more towards the sides of the head. The ostrich can see behind him, and fish can see in any direction without apparent limit. Man can only see indistinctly; and though he can move his eyes rapidly, he can see distinctly but a small portion of any object at a time, yet he sees with both eyes simultaneously a single object, because the two lines of vision unite at a single point, and as the two images cover each other we perceive only one image. Beyond or within this point of meeting the vision is indistinct, but the angle of convergence is always varied according to the distance of the object. If we hold up a penholder in front of us, and in a line with any other defined object—say an ink-bottle—we can see the penholder distinctly, and the ink-bottle indistinctly, as two images. If we then look at the ink-bottle we shall see it single, while the penholder will appear double, but with imperfect outlines.

      Fig. 147.—The Stereoscope.

      Fig. 148.—Mode of taking photograph for Stereoscope.

      Again, if we look at a box both eyes will see it equally well, but the right eye will see a little more on its right side, and the left eye on the left. It is on this principle that the Stereoscope is constructed. Sir Charles Wheatstone was the inventor, and the instrument may be thus described:—Two pictures are taken by photography—one as the landscape is seen by the right eye, the other as it is viewed by the left; the points of view thus differing slightly. When both eyes are simultaneously applied to the instrument the view is seen exactly as it would appear to the beholder at the actual place it represents. The views are taken singly; one side at one time, and another after, as in the camera (fig. 148). A is the first view; B is kept dark; C is the shutter for A. There are Reflecting and Refracting Stereoscopes. In the former a mirror reflects the image into each eye; in the latter the views are pasted on a card, side by side, and looked at through prismatic lenses. The principles of Binocular Vision have been applied to the Microscope.

      In foregoing chapters we have given many examples with diagrams of the temporary impressions made upon the retina of the eye. It is a fact that a wheel revolving at a great rate will appear to be standing still when suddenly illuminated by a flash of lightning, because the eye has not time to take in the motion in the instant of time, for the spokes of the wheel are not moving fast enough to convey the impression of motion in that half second to the eye; yet the perfect outline of the wheel is distinctly visible.

      Indeed, distinct vision can be exercised in a very small fraction of a second. It was calculated by Professor Rood, and proved by experiment, that forty billionths of a second is sufficient time for the eye to distinguish letters on a printed page. It this instance the illuminating power was an electric spark from a Leyden Jar.

      We have remarked upon the distinctness with which we can see an object when we direct our gaze upon it, and this appears a self-evident proposition; but have any of our readers remarked the curious fact that when they want to see a faint and particular star in the sky it will at once disappear when they gaze at it? The best way to see such very faint orbs as this is to look away from them—a little to one side or the other—and then the tiny point will become visible again to the eye. There is also a degree of phosphorescence in the eye, which any one who receives a blow upon that organ will readily admit. Even a simple pressure on the closed lid will show us a circle of light and “colours like a peacock’s tail,” as the great Newton expressed it. There are many occasions in which light is perceived in the eye—generally the result of muscular action; and the Irish term to “knock fire out of my eye” is founded upon philosophical facts.

      We are many of us aware of “spots” on our eyes when our digestion is out of order, and the inability of the eye to see figures distinctly in a faint light—within a proper seeing distance, too—has often given rise to the “ghost.” These shadowy forms are nothing more than affections of the eye, and, as well remarked in Brewster’s Letters on Natural Magic, “are always white because no other colour can be seen.” The light is not sufficiently strong to enable the person to see distinctly; and as the eye passes from side to side, and strives to take in the figure, it naturally seems shadowy and indistinct, and appears to move as our eyes move. “When the eye dimly descries an inanimate object whose different parts reflect different degrees of light, its brighter parts may enable the spectator to keep up a continued view of it; but the disappearance and reappearance of its fainter parts, and the change of shape which ensues, will necessarily give it the semblance of a living form; and if it occupies a position which is unapproachable, and where animate objects cannot find their way, the mind will soon transfer to it a supernatural existence. In like manner a human figure shadowed forth in a feeble twilight may undergo similar changes, and after being distinctly seen while it is in a situation favourable for receiving and reflecting light, it may suddenly disappear in a position before and within the reach of the observer’s eye; and if this evanescence takes place in a path or roadway where there is no sideway by which the figure could escape, it is not easy for an ordinary mind to efface the impression which it cannot fail to receive.” This will account for many so-called “ghosts.”

      

      Accidental colours, or ocular spectra, are, so to speak, illusions, and differently-coloured objects will, when our gaze is turned from them, give us different “spectra” or images. For instance, a violet object will, when we turn to a sheet of white paper, give us a yellow “spectrum”; orange will be blue; black and white will change