(D) ILLUMINATING APPARATUS (cont.)
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 off, 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 "frustules" of Diatontaceae. And it is one advantage 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 off 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 60º), and the angle of the objective is large (say 90º), 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 achromatic condensers of wide aperture can be thus used with objectives of 120º angle.
An excellent black-ground illumination is also given by the parabolic 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 representing the paraboloid. The parallel rays r, r´, r", 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 "bulls-eye" so adjusted as to produce this effect. There are many cases, however, 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 "immersion-paraboloid," 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.
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