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Stereoscopes 826

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STEREOSCOPES 826. Viewing Stereograms with the Naked Eye. By making the axes of the eye diverge, some people can view normal stereoscopic pairs with the naked eye, provided the distance between the principal points is little different from the separation of the eves. Such viewing is made easier by holding a piece of cardboard between the two pictures perpendicular to their common plane.

It is generally easier, by causing the axes of the eyes to converge (as in convergent strabism), to see in relief a pair which, when viewed nor mally, has the characteristics of a pseudoscopic pair, the left perspective being on the right, and vice versa. This kind of viewing is made easier by holding a mask at some distance from the eyes, only allowing one eye to see the single image which corresponds to it (Elliott and Waterston, 1857).

For example, to view at about loin., the two perspectives of the regular dodecahedron repro duced in Fig. 206 (M. Miet, 1921) one must hold at about 4 in. from the eyes a card with an opening of about I; in. square.' Examined with a binocular stereoscope (Fig. 208), this same drawing appears to be a dode cahedron, but not regular, in which the near faces will be those which appeared farthest when examined with the naked eye.

Remember, however, that the viewing of a stereogram with the naked eye by divergence or convergence of the ocular axes 2 is a great strain, so that it is impossible to make a complete study of a series of stereograms or to examine more than a few at a time.

827. Stereoscopes with Convergent Eyepieces. Most of the stereoscopes used for amateur stereoscopy and for some scientific and technical applications are derived from the prismatic lens stereoscope of Sir David Brewster its original form this instrument used as eye pieces two convergent lenses having their centres outside the centres of the two eyes (often reduced to two half-lenses), the separation of the centres being about 25 per cent greater than the separa tion of the eyes. Under these conditions one can view without fatigue a stereogram in which the separation of the principal points is about 15 per cent greater than the separation of the eyes, and may thus reach about 3 A " binocular stereoscope " is represented in Fig.

208 (A. I3uguet, 1891). This is the simplest form of stereoscope, useful for examining stereograms printed in a book or periodical. In Fig. 209 the paths of the rays are indicated in such a stereo scope, for the case of a near point of the recon structed object (the ocular axes-are convergent).

The eccentricity of the eyepieces increases the inevitable aberrations met with in the use of uncorrected or partially corrected lenses, the more so since the lenses must be of the shortest focal lengths compatible with covering the wide angle of the field ; and the shorter the focus the greater the aberrations due to the eccentricity of the eyepieces. It is therefore an advantage to have a stereoscope in which the separation of the eyepieces can be adjusted.

Some years ago cameras giving images farther apart than the mean separation of the eyes were almost completely abandoned in amateur stereo scopy. Under these conditions the separation of the principal points is equal to the mean separation of the eyes, and the eyepieces can We cannot go into all the variations met with in practice, and must refer the reader to the catalogues. However, we may mention the stereoscope in jurnelle (conical box) form, which is excellent for viewing transparencies,' and, by adjusting a movable shutter fitted with a mirror, also for the viewing of stereograrns on paper.

generally be centred relatively to the eyes. How ever, individuals having eyes abnormally close together require the eyepieces farther apart than those adjusted for eyes having a separation at least equal to the mean. Further, an observer unaccustomed to stereoscopic viewing can accommodate his eyes more easily to eyepieces having a greater separation than those which will suit him, after he has had some practice in stereoscopic viewing. For these two reasons stereoscopes are to be preferred which include the means of regulating the separation of the eyepieces,' besides the usual power of making the images coincide.

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