Online Encyclopedia

Search over 40,000 articles from the original, classic Encyclopedia Britannica, 11th Edition.

BINOCULAR

Online Encyclopedia
Originally appearing in Volume V18, Page 404 of the 1911 Encyclopedia Britannica.
» Make a correction to this article.
» Add information or comments to this article.
Spread the word: del.icio.us del.icio.us it!

BINOCULAR See also:

INSTRUMENTS The stereoscopic See also:microscope is the most suitable for finding out the space taken up by the See also:separate parts of a preparation. (See also BINOCULAR INSTRUMENTS and STEREOSCOPY.) The observer has a stereoscopic impression of an See also:object, when different See also:perspective representations are presented to both eyes, which, through the See also:action of the central See also:nerve See also:system, resolve into one impression. One way of receiving a stereoscopic impression through a micro-See also:scope is by fixing an apparatus as directly as possible above the last See also:lens of the microscopic See also:objective, which divides the rays passing out and directs See also:half into each eyepiece. The half cones of rays have now semicircular sections, the diaphragms having the same See also:form. The cones must be so directed through the divided system that the two exit pupils correspond to the interpupillary distance of the observer. The distance of the centres of the semicircular entrance pupils and their distance from the object regulates the difference of the two perspective representations, which are presented one to the right See also:eye and one to the See also:left. If the perspective centres See also: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 See also:hand, a very much exaggerated stereoscopic effect can be derived from See also:short focused systems of wide See also:aperture. On See also:account of the slight See also:depth See also:definition, short focused systems of wide aperture are not at all specially suitable for stereoscopic observation, because the possibility of observing See also:objects taking up a See also:good See also:deal of space is too limited when such systems are used. See also:Professor J. L. Riddell (Quart.

Journ. Micros, 1853, p. 236; 1854, pp. 18–24) published an arrangement of prisms, which, however, imparted a pseudomorphous impression if See also:

image-forming oculars were not used, and in 1854 a second system I! (fig. 48), essentially a See also:Wheatstone pseudoscope, added just above the objective. This gave an orthoscopic image even in See also:ordinary eye-pieces. By adopting right-angled reflection-prisms above the eyepiece he completely erected the image. See also:Stephenson's stereoscopic microscope (fig. 58, See also:Plate) resembles this apparatus in all essentials. A construction of prisms by Nachet is now almost forgotten, while on the contrary an extremely See also:simple dividing See also:prism published by Wenham (Lond. Micros.

See also:

Soc., 1861, i. 109) has been exception-ally well attested in practice. It is more easily used than any other apparatus (see BINOCULAR INSTRUMENTS, fig. 8). A reflec- tion-prism (fig. 49) in a setting is placed above the last See also:surface of the objective and divides the exit rays. The See also:group of rays coming from the left half of the objective can continue its way without hindrance to the right eye. The group of rays coming from the right half of the objective is reflected twice in the prism and directed to the left eye. The See also:tube containing the left eyepiece is a little inclined to-wards the right tube, which is perpendicular. It can be adapted to the interpupillary distance by changing the tube slide. If it is desired to use the See also:instrument as a monocular, the setting with the prism at the See also:lower end of the tube is taken away. A second manner of making stereoscopic observations employs stereoscopic eyepieces.

The first of such eyepieces was proposed by R. B. Tolles.1 He realized that the See also:

division of the cones of rays by prisms could only be satisfactorily performed if the prism was placed in the position of the exit See also:pupil of the objective or in the position of the real image of this exit pupil. He employed a Nachet See also:combination of prisms and placed the dividing prism at the spot where a See also:special See also:reversing system formed a real image of the exit pupil of the objective. A second stereoscopic eyepiece was devised by A. Prazmowski who substituted a Wenham diffracting division prism at the position of the real image of the exit pupil of the objective formed by a reversing system. The newest form of a stereoscopic microscope resembles the See also:oldest in so far as two completely separate microscopes. are used. In the oldest microscope by Cherubin d'See also:Orleans the observer receives a pseudoscopic impression in See also:con-sequence of the reversed image. This defect has been avoided in the instruments constructed in the Zeiss factory (fig. 59, Plate) at the instigation of the See also:American zoologist H. S. See also:Greenough.

The system of Porro prisms employed affords a convenient method of adapting the ends of the eyepieces to the interpupillary distance. The two tubes are inclined to one another at an See also:

angle of about 14°. The microscope is only intended for slight magnifications. The possibility already suggested of using both eyes for observing without having a stereoscopic impression, is often regarded as a See also:great See also:advantage. Binocular microscopes have therefore been constructed on this See also:plan. Such a combination of prisms was used by Wenham, who placed it directly behind the last objective lens. As a See also:rule this arrangement of prisms can be exchanged for the Wenham stereoscopic reflection-prisms. A second See also:kind of dividing prism which directs the entire course of rays to both eyes, and thus produces identical images, was used by See also:Powell and Lealand (fig.. 5o). Every See also:ray is divided into a re- flected and a refracted portion on the front See also:side of a parallel plate. Whilst the refracted portion after leaving the plate continues its way in the same direction, displaced a little to one side, the reflected portion is directed into the side tube by a reflection- prism. With these microscopes, which are not stereoscopic, objectives of any See also:power can be used.

The surfaces of the dividing prisms must be very exact, so that no deterioration of the image may arise from them. A microscope for two eyes can also be obtained by employing the See also:

Abbe stereoscopic eyepiece. By the supplementary use of one of Wenham's prisms every ray is analysed into a more powerful refracted and a weaker reflected one. The same image can be presented to each eye by using this eyepiece also. No stereoscopic impression is then See also:felt. It is brought about by placing special semicircular dia- phragms in the See also:plane of the exit pupil of the microscope. By 1 R. B. Tolles, See also:Sill. Journ. (1865), xxxix. 212; Journ.

See also:

Roy. Micr. Soc. (1890), pt. i. p. 383.turning the diaphragms 18o° See also:round the See also:optical See also:axis, the orthoscopic impression can be changed into the pseudoscopic. The See also:mechanical arrangement of the eyepiece is such that the distance of the two exit pupils can be adjusted to the interpupillary distance.

End of Article: BINOCULAR

Additional information and Comments

There are no comments yet for this article.
» Add information or comments to this article.
Please link directly to this article:
Highlight the code below, right click, and select "copy." Then paste it into your website, email, or other HTML.
Site content, images, and layout Copyright © 2006 - Net Industries, worldwide.
Do not copy, download, transfer, or otherwise replicate the site content in whole or in part.

Links to articles and home page are always encouraged.

[back]
BINNEY, THOMAS (1798-1874) 		[next]
BINOCULAR INSTRUMENT

Site © 2006 - Net Industries