Page:EB1911 - Volume 18.djvu/417

 the stage, a black or white plate, forming a dark or light background, can be swung underneath the specimen.

When the recognition of the arrangement in space of small objects is desired a stereoscopic lens can be used. In most cases refracting and reflecting systems are arranged so that the natural interpupillary distance is reduced. 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 s).

The view held by early opticians, that a compound microscope could never produce such good images as an instrument of the simple type, has proved erroneous; and the principal attention of modern opticians has been directed to the compound instrument. Although we now know how the errors of lenses may be corrected, and how the simple microscope may be improved, this instrument remains with relatively feeble magnification, and to obtain stronger magnifications the compound form is necessary.

History of the Compound Microscope.—The arrangement of two lenses so that small objects can be seen magnified followed soon after the discovery of the telescope. The first compound microscope (discovered probably by the Middelburg lens-grinders, Johann and Zacharias Janssen about 1590) was a combination of a strong biconvex with a still stronger biconcave lens; it had thus, as well as the first telescope, a negative eyepiece. In 1646 Fontana described a microscope which had a positive eyepiece. The development of the compound microscope essentially depends on the improvement of the objective; but no distinct improvement was made in its construction in the two centuries following the discovery. In 1668 the Italian Divini employed several doublets, i.e. pairs of plano-convex lenses, and his example was followed by Griendl von Ach. But even with such moderate magnification as these instruments permitted many faults were apparent. A microscope, using concave mirrors, was proposed in 1672 by Sir Isaac Newton; and he was succeeded by Barker, R. Smith, B. Martin, D. Brewster, and, above all, Amici. More recently these catadioptric microscopes were disregarded because they yielded unfavourable results. From 1830 onwards many improvements were made in the microscope objective; these may be best followed from a discussion of the faults of the image.

Position and Size of Image:—In most microscopic observations the object is mounted on a plane glass plate or slide about 0·06 in. thick, embedded in a liquid such as water, glycerine or Canada balsam, and covered with a plane glass plate of about 0·008 to 0·006 in. thick, called the cover-slip. If we consider the production of the image of an object of this kind by the two positive systems of a compound microscope shown in fig. 13, the objective L1 forms a real magnified image O′O1′; the object OO1, must therefore lie somewhat in front of the front focus F1 the objective. Let OO1＝y, O′O1′＝y&#8202;′, the focal distance of the image F1′O′＝ and the image-side focal length f1′, then the magnification

The distance A is called the “optical tube length.”

Weak and strong microscope objectives act differently. Weak systems act like photographic objectives. In this case the optical tube length may be altered within fixed limits without spoiling the image; at the same time the objective magnification M is also altered. This change is usually effected by mounting the objective and eyepiece on two telescoping tubes, so that by drawing apart or pushing in the tube length is increased or diminished at will. For strong objectives there is, however, only one optical tube length in which it is possible to obtain a good image by means of wide pencils, any alteration of the tube length involving a considerable spoiling of the image. This limitation is examined; below.

When forming an image by a microscope objective it often happens that the transparent media bounding the system have different optical properties. A series of objectives with short focal lengths are available, which permit the placing of a liquid between the cover-slip and the front lens of the objective; such lenses are known as “immersion systems”; objectives bounded on both sides by air are called “dry systems.” The immersion liquids in common use are water, glycerine, cedar-wood oil, monobromnaphthalene, &c. Immersion systems in which the embedding liquid, cover-slip, immersion-liquid and front lens have equal refractive indices are called “homogeneous immersion systems.” In immersion systems the object-side focal length is greater than the image-side focal length. Nothing is altered as to objective magnification, however, as the first surface is plane, and the employment of the immersion means that the value of f1′ is unaltered.

If we assume that a normal eye observes the image through the eyepiece, the eyepiece must project a distant image from the real image produced by the objective. This is the case if the image O′O1′ lies in the front focal plane of the eyepiece. In this case the optical tube length equals the distance of the adjacent focal planes of the two systems, which equals the distance of the image-side focus of the objective F1′ from the object-side focus of the eyepiece F2. The image viewed through the eyepiece appears then to the observer under the angle w″, and as with the single microscope

where f&#8202;′2 is the image-side focal length of the eyepiece.