Efforts have been made of late years to improve the available methods of representing ground, especially in Switzerland, but the so-called stereoscopic or relief maps produced by F.
In the British service half of the stereoscopic scissors-telescope used in rangefinders was frequently employed as a periscope.
When the recognition of the arrangement in space of small objects is desired a stereoscopic lens can be used.
Stereoscopic lenses can never be powerful systems, for the main idea is the recognition of the depth of objects, so that only systems having a sufficient depth of definition can be utilized.
Very often such stereoscopic lenses, owing to faulty construction, give a false idea of space, ignoring the errors which are due to the alteration of the inter-pupillary distance and the visual angles belonging to the principal rays at the object-side (see Binocular Instruments).
Binocular Instruments The stereoscopic microscope is the most suitable for finding out the space taken up by the separate parts of a preparation.
(See also Binocular Instruments and Stereoscopy.) The observer has a stereoscopic impression of an object, when different perspective representations are presented to both eyes, which, through the action of the central nerve system, resolve into one impression.
One way of receiving a stereoscopic impression through a microscope is by fixing an apparatus as directly as possible above the last lens of the microscopic objective, which divides the rays passing out and directs half into each eyepiece.
If the perspective centres lie too near one another in the object-space, as may happen with slightly opened and weak systems, the difference of the perspective is then too slight to make any real stereoscopic impression.
On the other hand, a very much exaggerated stereoscopic effect can be derived from short focused ?
On account of the slight depth definition, short focused systems of wide aperture are not at all specially suitable for stereoscopic observation, because the possibility of observing objects taking up a good deal of space is too limited when such systems are used.
Stephenson's stereoscopic microscope (fig.
A second manner of making stereoscopic observations employs stereoscopic eyepieces.
A second stereoscopic eyepiece was devised by A.
The newest form of a stereoscopic microscope resembles the oldest in so far as two completely separate microscopes are used.
The possibility already suggested of using both eyes for observing without having a stereoscopic impression, is often regarded as a great advantage.
As a rule this arrangement of prisms can be exchanged for the Wenham stereoscopic reflection-prisms.
With these microscopes, which are not stereoscopic, objectives of any power can be used.
A microscope for two eyes can also be obtained by employing the Abbe stereoscopic eyepiece.
No stereoscopic impression is then felt.
Binocular instruments should aid the natural spatial or stereoscopic vision, or make it possible if the eyes fail.
Such an impression could not possibly be stereoscopic, and these experiments, FIG.
Led to the construction of a non-stereoscopic binocular microscope.
At a later date attempts were made to separate the two halves of the objective by modifying the eye-piece; this led to the construction of stereoscopic eye-pieces, initiated by R.
Abbe and C. Pulfrich), constructed the first stereoscopic range-finder suitable for practical use.