The light, on the whole, most suitable for this method of illumination is lamp, the rays of which should in all cases be rendered more parallel by means of a large plano-convex lens, or condenser.
Fig. 146.—Wenham’s Hemispherical Lens.
Wenham’s Immersion Condenser.—Mr. Wenham, in the year 1856, described various forms of oblique illuminators, one of which was an immersion; a simple right-angled prism, connected by a fluid medium of oil of cloves. This, however, was abandoned for a nearly hemispherical lens connected with the slide, and although an improvement, did not touch the point of excellence Mr. Wenham was looking for. Ultimately he adopted a semi-circular disc of glass of the exact form and size represented in the drawing, [Fig. 146], having a quarter-inch radius, with a well-polished rounded edge, the sides being grasped by a simple kind of open clip attached to the sub-stage, the fluid medium used for connecting the upper surface with the slide being either water, glycerine, or oil; an increase of oblique illumination being obtained by swinging the ordinary mirror sideways. By means of an illuminator of the kind difficult objects mounted in balsam are resolved. This simple piece of glass collects and concentrates light in a marvellous manner, and is by no means a bad substitute for some of the more costly forms of achromatic condenser. It can be used either in fluid contact with the slide, or dry, as an ordinary condenser.
Mr. Wenham subsequently contrived a small truncated glass paraboloid, for use in fluid contact with the slide; water, glycerine, oil, or other substance being employed as a contact medium. The rays of light in this illuminator, being internally reflected from a convex surface of glass, impinge obliquely on the under surface of the slide, and are transmitted by the fluid uniting medium, and internally reflected from the upper surface of the cover-glass to the objective. To use the reflex illuminator efficiently it must be racked up to a level with the stage. The centre of rotation is then set true by a dot on the fitting, seen with a low power, a drop of water is then placed on the top, and upon this the slide is laid. Minute objects on the slide must be found either by the aid of a low power, by their greater brilliancy, or by rotating the illuminator; the effect on the podura scale is superb, the whole scale appearing dotted with bright blue spots in a zig-zag direction. Objects for this illuminator should be especially selected and mounted.
Fig. 147.—The Amici Prism.
The Amici Prism, originally designed for oblique illumination, consists of a flattened triangular glass prism, the two narrower sides of which are slightly convex, while the third or broadest side forms the reflecting surface. When properly used, it is capable of transmitting a very oblique pencil of light. The prism is either mounted, as in [Fig. 147], for slipping into the fitting of the sub-stage, or on an independent stand, as arranged for Powell’s microscope, page 85, [Fig. 56].
Method of Employing the Achromatic Condenser to the Greatest Advantage.
Its Illumination.—Good daylight is the best for general work. The microscope should be placed near a window with a northern aspect. Direct sunlight should never be utilised; the best light is that reflected from a white cloud. A good paraffin lamp is the most serviceable artificial source of light, and it is quite under control. As an illuminant more often brought into requisition in the smoky atmosphere of towns, the paraffin lamp is on the whole the handiest and the most useful. If gas-light can be brought into use as suggested for micro-photography, with the incandescent mantle, it will be found to be the purest and best form of artificial illumination for the microscope. Among paraffin lamps those constructed by Baker and Swift are all that can be desired.[28]