701. Bull's-Eye Condenser

Bull's-Eye Condenser. This useful piece of apparatus consists of a Piano-Convex lens mounted on an arm sliding on a pillar so that with a little manipulation any required position of the lens may be attained. One of the purposes of this accessory is to illuminate an opaque object by concentrating the light from any source upon the specimen which could not, owing to its opacity, be viewed by transmitted light from the mirror underneath. The bull's-eye condenser is also used to concentrate the rays of light upon the mirror when using artificial light with a high-power objective. Where gaslight is used as the il-luminant, the Welsbach gas lamp with bull's-eye lens will be found an improvement over the bull's-eye condenser, as it supplies a more concentrated light.

702. Eye-Pieces

Eye-Pieces. The eye-piece (see F, Illustration No. 120) consists of a couple of Piano-Convex lenses mounted in a metal tube, the lower section fitting flush with the metal tube, while the upper section is made with a milled edge and projects over the tube, thus permitting of the tube slipping into the microscope barrel and resting upon the milled head of the upper section. The Nos. 1 and 2 eye-pieces are marked according to their optical value. The one marked 1 has an initial magnification of ten diameters; the one marked 2 has five diameters. The low-power eye-piece (No. 2) is the most useful to the student and gives excellent results when used with the photo-micro-graphic camera.

703. Use Of The Yellow Screen

Use Of The Yellow Screen. Owing to the fact that many slides contain different shades of the same color, you will require a specially prepared plate or color-screen, which assists in rendering colors in their relative values to each other. For example, some specimens show several shades of red. With a specimen stained from a deep red to a delicate pink, it is evident that in exposing to obtain detail in the dark parts, you will inevitably obliterate the more delicate portions unless some restrainer is employed; hence the use of the yellow screen.

704. The yellow screen, according to its density, corrects the delicate colors; i. e., the screen restrains the action of light upon these colors during the exposure of the more opaque colors, thus giving a more uniform rendering of the different tones. This screen is usually placed just in front of the instrument and supported by the stage. It may be placed anywhere in the path of the light. The exact distance from the stage is of no particular importance, but it simplifies matters to have it lean against the stage, for then you are absolutely certain of cutting out any white light that is not wanted. (See Fig. 1 of Illustration No. 122, which shows the instrument in use for daylight work.)

705. While these filters or screens can be purchased already colored, yet one can very easily prepare their own by immersing two unexposed 5x7 plates in the acid hypo fixing bath, allowing them to remain in this solution until all milkiness or bromide of silver, has disappeared; then wash for half an hour in running water, after which place them in the rack to dry. When dried, immerse the plates in a saturated solution of picric acid for ten minutes, and without washing place them in the negative rack to dry. When dried, place the two film surfaces together and bind the edges with passe-partout paper, when they are ready for use.