The quarter- or semi-lens may also be used in the binocular camera. And here we may state the advantage of dividing the lenses. Whole lenses were originally used ; but as the outer half of each lens is useless, as the eyes only look through the inner halves; moreover, as it is impossible to give two lenses precisely the same focal length and magnifying power, it was found to be more accurate, and even cheaper, to cut each lens into halves or quarters, and to shape each half or quarter into a round disc, with the thin part of each turned inwards in the instrument. In this way a single lens could be made Into one semi-lens or two quarter-lens stereoscopes. It is evident that these portions, cut from the same lens, must have the same focal length and magnifying power. It is by means of these semi- or quarter-lenses that the stereoscopic effect is produced, though they do not themselves pro duce that effect. What they accomplish is the transference of the two dissimilar pictures or stereographs to a middle point. The union Of these two pictures, or their superposition on that middle point, produces the stereoscopic effect. [Sicily.] The half- or quarter-lenses are placed 24 inches apart, corresponding to the distance between the eyes.
Among the various forms of the instrument, we may mention Smith and Beck's achromatic stereoscope ; the reflecting stereoscope ; Claudet's stercomonoscope ; and Skaife's pistolograph, consisting of a combination of lenses of small size (one inch in diameter, and the focal length of the combination one inch). In this last instrument the thickness of the glass through which the light passes is small, and hence the actinic rays are 'so powerful that a photograph may be taken almost in an instant, and is not liable to the errors which the use of large lenses occasions. The small pictures, or pistolograms, as they are called, may be magnified by an enlarging camera. The small picture may also be inclosed between two plates of glass, and raised to a tem perature sufficient to fuse the three glasses into one, effectually pro tecting the picture from the presence of the air, and forming what is called a chromo-crystal.
From what has been said, it will be gathered that the truthfulness of the stereoscopic picture must depend mainly on the character of the dissimilar pictures or stereographs. Stereographic portraits are usually
taken with cameras contrived for the purpose. In order to take stereo graphs of landscapes, buildings, statuary, &c., the ordinary landscape camera is employed ; the camera being removed, after the first picturo is taken, to a position parallel to that just occupied, and at en equal dis tance from the principal object, but more or less distant from the first position in proportion to the distance from the object to be repre sented. The stereoscopic angle, as it is called, has been laid down at 1 in 25 for objects 50 feet or more distant ; some have even recom mended that the camera should be removed to a distanco of 4 feet in order to take views of an object only 20 feet distance. But the effect of such an arrangement is obviously to make one picture represent much more of the right side, the other more of the left side, of an object falliug within the field of vision, than could be seen by a person standing, say midway, between the two positions. And the two pk tures so taken must, when united in the stereoscope, present an exagge rated and therefore untrue representation. In fact there will be, what is so commonly seen in the stereoscope, an unnatural appearance of separation between the chief object and the accessories. You see round the figure in fact, just as in life you see round a statuette or small model ; and hence there arises that detached model-like appear ance which is often, and very properly, objected to in stereoscopic repro sentatimul. What the stereoscope ought to show is, the representation of an object or objects in nearly the same relative solidity, relief, and separation as the reality possesses; and that is what the stereoscope would exhibit if the stereographs were taken, as they ought to be, and as the most successful (though not the most popular) are taken, from positions little, if at all, exceeding that of the eyes apart. The great importance of strict accuracy in views of countries beyond the reach of the ordinary traveller, of antiquities, objects of special scientific and arclueological interest, &c., will be at once acknowledged; and the value of the stereoscope for affording such representations in their greatest attainable perfection is daily becoming more apparent