Page:The American Cyclopædia (1879) Volume XI.djvu/537

 MICROSCOPE 519 proceed up the secondary tube 2, as shown in fig. 3, B. To obtain stereoscopic effect.it is absolutely necessary, on account of the cross- ing of the rays of the pencils in the objective, that those rays which have emanated from the right-hand half of the cut pencil should enter the left eye, and vice versa. If this does not take place, we shall have, instead of stereo- scopic relief, a pseudoscopic effect; that is, elevations will appear as depressions and de- pressions as elevations. This was the case with Prof. Riddell's invention and with the first plan adopted by Mr. Wenham. In Mr. Wenham's recent invention, which we have just described, this is avoided by causing the rays reflected from the face c of the prism to cross those going directly up the tube, so that the right eye receives the left half of the pen- cil, and the left eye the right half. The dis- tance between the oculars 1 and 2, fig. 3, A, is adapted to different eyes by means of racks and a pinion, by which they are moved inward or outward. There are two great advantages in using the binocular microscope : first, the eyes are subjected to equal work, and thereby are not so liable to fatigue and injury ; secondly, with low powers we obtain an insight into the three dimensions of a body, for in the stereo- scopic binocular we see into the depth of a body as well as view its length and breadth. This is very important in the study of tissues, as we see how they are arranged in order of distance from the surface of the body. The usual accessories accompanying the microscope are : an achromatic condenser, a bull's-eye con- denser, small condenser, stage and eye-piece micrometers, polarizing apparatus, camera lu- cida, animalcule cage, stage forceps, glass para- bola, erector, Lieberktihns and dark wells, frog plate, &c. The name of achromatic condenser has been given to an illuminating apparatus consisting of an achromatic objective of large angle, furnished with a wheel of diaphragms and central stops; when the latter are used, oblique illumination is obtained. This con- denser is exceedingly useful in the ordinary studies by the microscope. With the proper adjustment it affords a fine achromatic illumi- nation, revealing the structure of the object with great beauty and clearness ; but in un- skilful hands it will be of little service. So far as oblique illumination is concerned, we prefer unilateral light, though some particu- lars of structure are best revealed by aid of the achromatic condenser. The condenser is placed below the main stage, and can be ad- justed by independent rackwork, so as to give the best illumination. The condenser of 170 angle, introduced by Powell and Lealand, is spoken of very highly by the English micro- scopists; the delicate markings of namcula rhomboides, when viewed with their ^ objec- tive, are said to be as well defined as those of pleuroslgma hippocampus viewed with a J. The achromatic condenser of Beck has an an- gle of about 100. When used with the central stops, the proper point for adjustment may be determined by removing the eye piece, after the objective has been brought into focus, looking down the tube, and moving the con- denser by means of its own rack and pinion, until the black stop appears sharply defined in the centre of the field, and a bright illuminated ring around it. If the stop is removed, the whole field will appear brightly illuminated, unless the objective be of much greater angle than the condenser. A condenser of greater angle than 100 cannot be used unless the ob- ject be placed upon very thin glass ; all diffi- cult test objects should be mounted in this manner on a slip of mahogany, or on a per- forated glass slide. The condenser itself, or the carrier, should have some means of adjust- ment to make its axis coincide with that of the objective ; the front set of lenses is gen- erally made to be removed, thus affording an illuminator of lower angle, to use with thicker glass and objectives of low power. In using the achromatic condenser by lamp light, the bull's-eye condenser is employed to render the rays parallel, and the plane side of the mir- ror is used. The bull's-eye condenser consists of a thick plano-convex lens, of short focus, mounted upon a stand so that it can be used for the illumination of opaque objects. Very excellent oblique illumination may be obtained by condensing the flame of a candle or lamp, placed about two feet from the stage, upon the under surface of the slide, the light being placed in front, and the stage slightly turned up to receive it ; or, the microscope being placed horizontal, the light may be off at one side, ac- cording to the obliquity desired. As the thick- ness of the stage will not ordinarily allow illu- mination at a much greater distance than 60 from the axis, a sub-stage, attached to the upper plate of the main stage, but carrying the slide entirely below it, is furnished by Messrs. Gru- now and by Powell and Lealand, and may be readily attached to any microscope ; this allows illumination of any obliquity. In using the bull's-eye condenser for this purpose with a lamp, the plane side must be presented toward the object; and if the flame be flat, its edge must be presented. The small condenser is used in the same way as the bull's-eye. Where very intense illumination is desired, the bull's- eye is placed near the source of illumination, with the plane side toward it, so as to render the rays nearly parallel, and then this beam of light is further condensed by the small con- denser. An achromatic lenticular prism of short focus is very valuable for illumination. The prism should be so mounted that it can be placed to reflect converging rays on the object at any angle, and in any position, both below and above the stage. Experience in its use will to a great extent supply the need of an achromatic condenser. The stage micrometer is a slip of glass ruled into -01" and -001 " of an inch; it is used in conjunction with the camera lucida, or to determine the value of the