FIG. 39. denser of this description he states to be suitable also for all ordinary purposes. ("Quart. Microsc. Journ.," vol. iii, p. 87.) Such an instrument, when its axis does not form a very large angle with that of the microscope, may receive its light from the plane mirror, especially if this be so mounted as to be capable of being turned considerably out of the visual axis; but when its position is too oblique for the light to be thus supplied to it, recourse must be had to rays either proceeding direct from their source (such as a lamp or a bright cloud), or directed at the requisite angle by a reflector placed in a suitable position. For this latter purpose, a rectangular prism (§ 57), mounted on a separate stand, will be found very convenient. By many observers, a combination of the reflecting and refracting powers of a prism is preferred, which causes the rays to be at once reflected by a plane surface, and concentrated by lenticular surfaces; so that the prism answers the purpose of mirror and condenser at the same time. Such a prism was first constructed by Amici; and it may be either mounted on a separate base, or attached to some part of the microscope-stand. The mounting adopted by Messrs. Smith and Beck, and shown in Fig. 39, is a very simple and convenient one; this consists in attaching the frame of the prism to a sliding bar, which works in dovetail grooves on the top of a cap that may be set on the cylindrical fitting beneath the stage; the slide serves for the regulation of the distance of the prism from the axis of the microscope, and consequently of the obliquity of the illumination; whilst its distance beneath the stage is adjusted by the rack-movement of the cylindrical fit- Amici's Prism for oblique illumination. ting. In this manner, an illuminating pencil of almost any degree of obliquity may be readily obtained; but there is no provision for the correction of its aberrations. Such a provision is afforded by the achromatic prism of Mr. Abraham (§ 57), which may be mounted in the manner just described. And the same object is attained by an arrangement devised by Mr. Grubb, a Dublin optician, of which Dr. Robinson, of Armagh, speaks very highly; the prism having its aberrations corrected for a lamp placed at a given distance in the plane of the stage; and being mounted in such a manner as to be capable of travelling (like Mr. Sollitt's condenser) through an angular range of as much as 120° ("Quart. Microsc. Journ.," vol. iii, p. 166). In all of these methods, the obliquity of the illumination is practically limited by the construction of the stage, and especially by the relation which its thickness bears to the diameter of its lower aperture. The thinner the stage, and the larger its lower aperture, the more oblique will be the rays which may be transmitted through it; and in admitting an extreme obliquity of illumination, the thin stage recently introduced into some of the best Microscopes (§§ 38, 39) possesses a great advantage over all whose thickness is greater. On the other hand, it is when the rays are most oblique, that the greatest advantage is gained by making them fall upon the object from every side in succession; and where this cannot be accomplished (as in the case of Nachet's prism) by the rotation of the illuminating apparatus, the rotatory movement must be given to the object. It is obvious that, for this purpose, a revolving stage which keeps the object constantly in the field (§ 37), is decidedly preferable to one which does not possess such a movement; but the means have not yet been found of obtaining this advantage without some sacrifice of the other. 61. Whenever the rays are directed with such obliquity, as not to be received into the object-glass at all, but are sufficiently retained by the object, to render it (so to speak) self-luminous, we have what is known as the black ground illumination; to which the attention of Microscopists generally was first drawn by the Rev. J. B. Reade, in the year 1838, although it had been practised some time before, not only by the Author but by several other observers. For low powers whose angular aperture is small, and for such objects as do not require any more special provision, a sufficiently good "black ground" illumination may be obtained by means of the concave mirror alone, especially when it is so mounted as to be capable of a more than ordinary degree of obliquity. In this manner it is often possible, not merely to bring into view features of structure that might not otherwise be distinguishable, but to see bodies of extreme transparency (such, for instance, as very minute Animalcules) that are not visible when the field is flooded (so to speak) by direct light; these presenting the beautiful spectacle of phosphorescent points rapidly sailing through a dark ocean. Where the mirror cannot be placed in a position oblique enough to give this effect, a black ground illumination sufficiently good for many purposes may be obtained by Mr. Reade's original method; which consisted in dispensing with the mirror altogether, and in placing the lamp and ordinary condensing-lens (§ 64) in such a position beneath and to one side of the stage, as to throw upon the under side of the object a pencil of rays too oblique to enter the object-glass after passing through it. Another very simple mode, which answers sufficiently well for low powers and for the larger objects which these are fitted to view, consists in the substitution, for the achromatic condenser, of a plano-convex lens of great convexity, forming a large segment of its sphere, with a central stop to cut off the direct rays; for the rays passing through the marginal portion of this Spotted Lens, being strongly refracted by its high curvature, are made to converge at an angle too wide for their entrance into an objective of moderate aperture, and thus the field is left dark; whilst all the light stopped by the object serves (as it were) to give it a luminosity of its own. Neither of the foregoing plans, however, will answer well for objectives of high power, having such large angles of aperture that the light must fall very obliquely to pass beyond them altogether. Thus if the pencil formed by the "spotted lens" have an angle of 60°, its rays will enter a 1-4th-inch objective of 70°, and the field will not be darkened. For obtaining a greater degree of obliquity, Mr. Wenham has contrived a Parabolic Speculum,' having its apex cut off, so that the object might be placed in the focus, to which all rays parallel to its axis are reflected; and the direct rays being checked by a stop placed behind it, the object is illuminated only by those which are reflected to it from all sides of the interior of the parabola at a very oblique angle. As the thickness of the glass slide on which the object is mounted, was found by Mr. W. to produce a very sensible aberration in the rays converging towards it, he interposed a meniscus lens, having such a curvature as to produce a counteracting aberration of an opposite kind. The circular opening at the bottom of the wide tube (Fig. 40) that carries the speculum, may be fitted with a diaphragm, adapted to cover any portion of it that may be desired; and by giving rotation to this diaphragm, rays of great obliquity may be made to fall upon the object from every azimuth in succession (§ 60). A like purpose was aimed at in the Annular Condenser of Mr. Shadbolt, which consists of a ring of glass, whose surface was so shaped as to present a prismatic section; the inclination of the outer side being such as to produce a total reflection of the rays impinging on it, and to direct these through the inner side of the ring, so as to fall at a very oblique angle upon the object, from every azimuth of the circle. A combination of both methods is adopted in the Parabolic Illuminator (Fig. 40), now supplied by Messrs. Smith and Beck; for this consists of a paraboloid of glass, resembling a cast of the interior of Mr. Wenham's parabolic speculum, but reflecting the rays which fall upon the outer surface of the glass, like Mr. Shadbolt's annular prism. It has the advantage of being more easily constructed than the parabolic speculum, and is little, if at all, inferior to it in performance; but it requires that an appropriate "stop" should be adapted to it, for each objective with which it is to be used: whilst in Mr. Wenham's speculum, the requisite adaptation for the angular aperture of the objective is made by altering the position of the stop by means of the central stem; the effect of which alteration is to cut off a larger and larger proportion of the least oblique rays, the more nearly the stop is approximated to the ob FIG. 40. Parabolic Illuminator. 1 "Transactions of the Microscopical Society" (1st Series), vol. iii, p. 85. ject; and thus to illuminate it more and more exclusively by those which meet at the widest angle. In using either of these illuminators, the rays which are made to fall upon them 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 reflection from the mirror, by the interposition of the "bull's eye" condenser (§ 64) so adjusted as to produce this effect. 62. For the exhibition of those classes of objects which are suitable for "black ground" illumination, and which are better seen by light sent into them from every azimuth, than they are by a pencil, however bright, incident in one direction only, no more simple, convenient, and efficient means could probably be found, than that which is afforded by the "spotted lens" for low powers, and by the "parabolic illuminator" for powers as high as 1-4th or 1-5th of an inch focus;-the use of the latter with the highest powers, being rendered disadvantageous by the great reduction in the amount of light, occasioned by the necessity for cutting off of all the rays reflected from the paraboloid, which fall upon the object within the limits of their angle of aperture. One of the great advantages of this kind of illumination consists in this that, as the light radiates from each part of the object as its proper source, instead of merely passing through it from a more remote source, its different parts are seen much more in their normal relations to one another, and it acquires far more of the aspect of solidity. The rationale of this is easily made apparent by holding up a glass vessel with a figured surface between one eye and a lamp or a window, so that it is seen by transmitted light alone; for the figures of its two surfaces are then so blended together to the eye, that unless their form and distribution be previously known, it can scarcely be said with certainty which markings belong to either. If, on the other hand, an opaque body be so placed behind the vessel, that no rays are transmitted directly through it, whilst it receives adequate illumination from the circumambient light, its form is clearly discerned, and the two surfaces are differentiated without the least difficulty. 63. Polarizing Apparatus.-In order to examine transparent objects by polarized light, it is necessary to employ some means of polarizing the rays before they pass through the object, and to apply to them, in some part of their course between the object and the eye, an analyzing medium. These two requirements may be provided for in different modes. The polarizer may be either a bundle of plates of thin glass, used in place of the mirror, and polarizing the rays by reflection; or it may be a "single image" or "Nicol" prism of Iceland Spar, which is so constructed as to transmit only one of the two rays into which a beam of ordinary light is made to divaricate on passing through this substance; or it may be a plate of Tourmaline, or one of the artificial tourmalines composed of the disulphate of iodine and quinine, now known by the designation of "Herapathite," after the name of their discoverer. Of these methods, the "Nicol" prism is the one generally preferred; the objection to the reflecting polarizer being, that it cannot be made to rotate; the tourmaline being undesirable, on account of the color which it imparts when sufficiently thick to produce an effective polarization; and the crystals of Herapathite being seldom obtained perfect, of sufficient size to afford a good illumination. The polarizing prism is usually fitted into a tube (Fig. 41, A, a) with a large milled head (c) at the bottom, by which it is made to rotate in a collar (6) that is attached to the microscope; this collar may be fitted to the under side of the stage-plate, or, where a secondary stage is provided, it may be attached to this; in the microscope of Messrs. Smith and Beck, it screws into the lower part (b) of a tube (Fig. 41, B) that slides into the "cylindrical fitting" beneath the stage (Fig. 29). The analyzer, which may be either a "Nicol" prism, a Tourmaline, or a crystal of Herapathite, is usually placed either in the interior of the microscope, or between the eye-piece and the eye. If it be a prism, it is mounted in a tube, which may either be screwed into the lower end of the body in the situation of the erector (Fig. 32), or may be fitted over the eye-piece in place of its ordinary cap (Fig. 42); in the former situation it has the advantage of not limiting the field, but it stops a considerable proportion of the light; in the latter, it detracts much less from the brightness of the image, but cuts off a good deal of the margin of the field. A plate of Tourmaline or Herapathite, if obtainable of sufficient size and freedom from color, has a decided advantage above the Nicol prism, as an analyzer, in being free from both these inconveniences; and it may be set in a cap which fits over the ordinary cap of the eye-piece. For bringing out certain effects of color by the use of Polarized light (Chap. XX), it is desirable to interpose a plate of Selenite beneath the polarizer and |