Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/288

Rh 272 MICROSCOPE illuminator, at any distance that is found to produce the best effects. In using this illuminator, the lamp should be placed at a distance of about 8 inches from the aperture; and, when the proper adjust ments have been made, the image of the flame should be seen upon the object. The illumination of the entire field, or the direction of the light more or less to either side of it, can easily be managed by the interposition of a small condensing lens placed at about the dis tance of its own focus from the lamp. The objects viewed by this mode of illumination with dry-front objectives are best uncovered, since, if they are covered with thin glass, so large a proportion of the light sent down upon them is reflected from the cover (especially when objectives of large angle of aperture are employed) that very little is seen of the objects beneath, unless their reflective power is very high. With immersion objectives, however, covered objects may be used. Another method of vertical illumination long since devised by Mr Tolles has recently been brought into notice by Professor W. A. Rogers of Boston (U. S.). It consists in the in troduction of a small rectangular prism at a short distance behind the front combination of the objective, so that parallel rays enter ing its vertical surface pass on between its parallel horizontal sur faces until they meet the inclined surface, by which they are reflected downwards. In passing through the front combination of the objective, they are deflected towards its axis ; but, as their angle of convergence is less than the angle of divergence of the rays proceeding from the object, the reflected rays &amp;gt;ill not meet in the focal point of the lens, but will be so distributed as to illuminate a sufficient area. By altering the extent to which the prism is pushed in, or by lifting or depressing its outer end by means of a milled-head screw, the field of illumination can be regulated. The working of this prism with immersion-objectives is stated by Mr Tolles to be peculiarly satisfactory. Black-Ground Illumination. There are certain classes of objects which, though sufficiently transparent to be seen with light trans mitted through them, are best viewed when illuminated by rays of such obliquity as not to pass directly into the objective, such a proportion of these rays being retained by the object as to render it self-luminous, when, all direct light being cut oft, the general field is perfectly dark. This method is particularly effective in the case of such delicate mineral structures as the siliceous tests of Poly- cystina and the &quot; frustules &quot; of Diatomaceie. And it is one ad vantage of this kind of illumination that it brings out with considerable effect the solid forms of objects suited to it, even when they are viewed monocularly. Two modes of providing this illumination are in use, each of which has its special advantages. One consists in placing a central stop either upon or immediately beneath a condenser of wide aperture, which shall cut oft all rays save those that, after passing through the object (as in fig. 20), diverge at an angle greater than that of the objective used ; so that, while the ground is darkened, the object is seen brightly standing out upon it. But if the divergence of the rays is but moderate (say 60), and the angle of the objective is large (say 90), the most divergent rays of the condenser will enter the mar ginal portion of the objective, and, the field not being darkened, the black-ground effect will not be produced. This method has the great convenience of allowing black -ground illumination to be substituted for the ordinary illumination under different powers, without any other change in the apparatus than the turning of a diaphragm-plate fitted with stops of different sizes suitable to the several apertures of the objectives ; and the modern achro matic condensers of wide aperture can be thus used with objectives of 120 angle. An excellent black-ground illumination is also given by the para bolic illuminator (fig. 19), originally worked out as a silvered speculum by Mr Wenham, but now made as a paraboloid of glass that reflects to its focus the rays which fall upon its internal surface. A diagrammatic section of this instrument, showing the course of the rays through it, is given in fig. 20, the shaded portion repre senting the paraboloid. The parallel rays r, r 1, r&quot;, entering its lower surface perpendicularly, pass on until they meet its parabolic surface, on which they fall at such an angle as to be totally reflected by it, and are all directed towards its focus F. The top of the paraboloid being ground out into a spherical curve of which F is the centre, the rays in emerging from it undergo no refraction, since each falls perpendicularly upon the part of the surface through which it passes. A stop placed at S prevents any of the rays reflected upwards by the mirror from passing to the object, which, being placed at F, is illuminated by the rays reflected into it from all sides of the paraboloid. Those rays which pass through it diverge again at various angles ; and if the least of these, GFH, be greater than the angle of aperture of the object-glass, none of them can enter it. The stop is attached to a stem of wire, which passes vertically through the paraboloid and terminates in a knob beneath, as shown in fig. 19 ; and by means of this it may be pushed upwards, so as to cut off the less divergent rays in their passage towards the object, thus giving a black -ground illumination with objectives of _an angle of aperture much wider than GFH. In using the paraboloid for delicate objects, the rays which are made to enter it should be parallel ; consequently the plane mirror should always be employed ; and when, instead of the parallel rays of daylight, we are obliged to use the diverging rays of a lamp, these should be rendered as parallel as possible, previously to their reflexion from the mirror, by the interposition of the &quot;bull s-eye&quot; so ad justed as to produce this effect. There are many cases, however, FIGS. 19, 20. Wenham s Parabolic Illuminator. in which the stronger light of the concave mirror is preferable. When it is desired that the light should fall on the object from one side only, the circular opening at the bottom of the wide tube that carries the paraboloid may be fitted with a diaphragm adapted to cover all but a certain portion of it ; and, by giving rotation to this diaphragm, rays of great obliquity may be made to fall upon the object from every azimuth in succession. In order to adapt this paraboloid to objectives of very wide angle of aperture, a special modification of it, originally devised by Mr Wenham, has been latterly reintroduced under the designation of &quot;immersion-paraboloid,&quot; with most excellent effect. This consists in making the top of the paraboloid flat instead of concave, and in interposing a film of glycerin between its surface and the under surface of the glass slide carrying the object. Only rays of such extreme obliquity are allowed to pass into the slide as would be totally reflected from its under surface if they fell upon it through air ; and, as these illuminate the object without passing into the objective, it can be thus examined under even the highest powers. BINOCULAR MICROSCOPES. Stereoscopic Binoculars. The admirable invention of the stereo scope by Professor Wheatstone has led to a general apprecia tion of the value of the conjoint use of both eyes, in conveying to the mind a conception of the solid forms of objects such as the use of either eye singly does not generate with the like certainty or effectiveness (see STEREOSCOPE). This conception is the product of the mental combination of the dissimilar perspective projections which our right and left retinae receive of any object that is suffi ciently near the eyes for the formation of two images that are sen sibly dissimilar. Now it is obvious that a similar difference must exist between the two perspective projections of any object in relief that are formed by the right and left halves of a microscopic ob jective and that this difference must increase with the angular aperture of the objective. And the fact of this difference may be easily made apparent experimentally, by adapting a semicircular &quot;stop&quot; to any objective of from 20 to 30 angle in such a manner that it can be turned so as to cover either its right or its left half ; for not only will the two images of any projecting object formed by the rays transmitted through the two uncovered halves be found sensibly different, but, if they be photographed or accurately drawn the &quot;pairing&quot; of their pictures in the stereoscope will bring out the form of the object in vivid relief. What is needed, therefore, to give the true stereoscope ;ffect to a binocular microscope is a means of so bisecting the cone of rays transmitted by the objective that its two lateral halves shall be transmitted the one to the right and the other to the left eye, and that the two images shall be crossed (the image formed by the right half of the objective being sent to the left eye, and that formed by the left half of the objective being sent to the right eye) in order to neutralize the reversing effect of the microscope itself. If this crossing does not take place, tfie effect will be rendered &quot; pseudoscopic, &quot; not &quot;orthosoopic,&quot; its projec tions becoming depressions, and its depressions being brought out as prominences. It was from a want of due appreciation of this fact that the earlier attempts at constructing a stereoscopic binocular gave representations of objects placed under it, not in their true orthoscopic, but in their pseudoscopic aspect. This was the case, for example, with the binocular microscope first devised by