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before the right-hand mirror, and the left eye before the left-hand mirror, and he must move the sliding pannels E, E" to or from him till the two reflected images coincide at the intersection of the optic axes, and form an image of the same apparent magnitude as each of the component pictures. The picture will, indeed, coincide when the sliding pannels are in a variety of different positions, and, consequently, when viewed under different inclinations of the optic axes, but there is only one position in which the binocular image will be immediately seen single, of its proper magnitude, and without fatigue to the eyes, because in this position only the ordinary relations between the magnitude of the pictures on the retina, the inclination of the optic axes, and the adaptation of the eye to distinct vision at different distances, are preserved. In all the experiments detailed in the present memoir I shall suppose these relations to remain undisturbed, and the optic axes to converge about six or eight inches before the eyes.
"If the pictures are all drawn to be seen with the same inclination of the optic axes the apparatus may be simplified by omitting the screw rl, and fixing the upright boards d, D' at the proper distance. The sliding pannels may also be dispensed with, and the drawings themselves be made to slide in the grooves."
The figures to which Mr. Wheatstone applied this instrument were pairs of outline representations of objects of three dimensions, such as a cube, a cone, the frustum of a square pyramid, which is shewn on one side of E, E1 in Fig. 10, and in other figures; and he employed them, as he observes, "for the purpose of illustration, for had either shading or colouring been introduced it might be supposed that the effect was wholly or in part due to these circumstances, whereas, by leaving them out of consideration, no room is left to doubt that the entire effect of relief is owing to the simultaneous perception of the two monocular projections, one on each retina."
"Careful attention," he adds, "would enable an artist to draw and paint the two component pictures, so as to present to the mind of the observer, in the resultant perception, perfect identity with the object represented. Flowers, crystals, busts, vases, instruments of various kinds, &c, might thus be represented, so as not to be distinguished by sight from the real objects themselves."
This expectation has never been realized, for it is obviously beyond the reach of the highest art to draw two copies of a flower or a bust with such accuracy of outline or colour as to produce " perfect identity," or anything approaching to it, " with the object represented."
Photography alone can furnish us with such representations of natural and artificial objects; and it is singular that neither Mr. Elliot nor Mr. Wheatstone should have availed themselves of the well-known photographic process of Mr. Wedgewood and Sir Humphry Davy, which, as Mr. Wedgewood remarks, wanted only " a method of preventing the unshaded parts of the delineation from being coloured by exposure to the day, to render the process as useful as it is elegant." When the two dissimilar photographs were taken they could have been used in the stereoscope in candle-light, or in faint day-light, till they disappeared, or permanent outlines of them might have been taken and coloured after nature.
Mr. Fox Talbot's beautiful process of producing permanent photographs was communicated to the Royal Society in January 1839, but no attempt was made till some years later to make it available for the stereoscope.
In a chapter on binocular pictures, and the method of executing them in order to reproduce, with perfect accuracy, the objects which they represent, we shall recur to this branch of the subject.
Upon obtaining one of these reflecting stereoscopes as made by the celebrated optician, Mr. Andrew Ross, I found it to be very ill adapted for the purpose of uniting dissimilar pictures, and to be imperfect in various respects. Its imperfections may be thus enumerated :—
1. It is a clumsy and unmanageable apparatus, rather than an instrument for general use. The one constructed for me was 16^ inches long, 6 inches broad, and 81 inches high.
2. The loss of light occasioned by reflection from the mirrors is very great. In all optical instruments where images are to be formed, and light is valuable, mirrors and specula have been discontinued. Reflecting microscopes have ceased to be used, but large telescopes, such as those of Sir W. and Sir John Herschel, Lord Rosse, and Mr. Lassel, were necessarily made on the reflecting principle, from the impossibility of obtaining plates of glass of sufficient size.
3. In using glass mirrors, of which the reflecting stereoscope is always made, we not only lose much more than half the light by the reflections from the glass and the metallic surface, and the absorbing power of the glass, but the images produced by reflection are made indistinct by the oblique incidence of the rays, which separates the image produced by the glass surface from the more brilliant image produced by the metallic surface.
4. In all reflections, as Sir Isaac Newton states, the errors are greater than in refraction. With glass mirrors in the stereoscope, we have four refractions in each mirror, and the light transmitted through twice the thickness of the glass, which lead to two sources of error.
5. Owing to the exposure of the eye and every part of the apparatus to light, the eye itself is unfitted for distinct vision, and the binocular pictures become indistinct, especially if they are Daguerreotypes,1 by reflecting the light incident from every part of the room upon their glass or metallic surface.
6. The reflecting stereoscope is inapplicable to the beautiful binocular slides which are now being taken for the lenticular stereoscope in every part of the world, and even if we cut in two those on paper and silver plate, they would give, in the reflecting instrument, converse pictures, the right-hand part of the picture being placed on the left-hand side, and vice versa.
7. With transparent binocular slides cut in two, we could obtain pictures by reflection that are not converse; but in using them, we would require to have two lights, one opposite each of the pictures, which can seldom be obtained in daylight, and which it is inconvenient to have at night.
Owing to these and other causes, the reflecting stereoscope never came into use, even after photography was capable of supplying binocular pictures.
As a set-off against these disadvantages, it has been averred that in the reflecting stereoscope we can use larger pictures, but this, as we shall shew in a future chapter, is altogether an erroneous assertion.
1 Mr. Wheatstone himself says, "that it is somewhat difficult to render the two Daguerreotypes equally visible."—Phil. Tram., 1852, p. 6.
Description of the Lenticular Stereoscope.
Having found that the reflecting stereoscope, when intended to produce accurate results, possessed the defects which I have described, and was ill fitted for general use, both from its size and its price, it occurred to me that the union of the dissimilar pictures could be better effected by means of lenses, and that a considerable magnifying power would be thus obtained, without any addition to the instrument.
If we suppose a, b, Fig. 11, to be two portraits,—A a portrait of a gentleman, as seen by the left eye of a person
viewing him at the proper distance and in the best position, and B his portrait as seen by the right eye, the purpose of the stereoscope is to place these two pictures, or rather their images, one above the other. The method of