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CH. IX]
MICRO-PROJECTION WITH AMPLIFIERS
229
§ 356. Amplifiers. — An amplifier is a concave lens or combination producing divergence instead of convergence of light rays, hence placing an amplifier in the path of the image-forming rays from the objective produces a larger image (fig. 126), and there is little loss in light. It should be made as great in diameter as the large tube (fig. 121) of the microscope will receive to avoid cutting down the field, and should be mounted in a short tube which can be easily slipped into a cloth-lined collar screwed into the end of the microscope tube (fig. 133).
The amplifiers most generally useful are of -5 and -10 diopters. The average increase in magnification given by the -5 diopter amplifier is 1.7 and that given by the -10 diopter is 2.5 (see § 356a).
Object
Objective
Microscooe Tube 122 x 46 mm. ***•*, ~~~~^-
FIG. 126. AMPLIFIER FOR PROJECTION.
Object The object to be projected.
Objective The projection objective.
Axis Optic axis of the apparatus.
A mplifier The concave lens diverging the rays from the objective and thus increasing the screen image.
Images The ones with broken lines show the images with a s diopter and a -10 diopter lens. The full lines show the image which the objective alone would give.
The microscope tube is 122 mm. (4.8 in.) long and 48 mm. (1.9 in) in diameter.
§ 356a, 403a. Diopter, Dioptre, Dioptry.— For spectacle lenses especiallv this is the unit of strength. It is the strength of a lens of i meter principal focus.
As the focal length of a lens varies inversely as its power, the focal length of a lens of 2 diopters is one-half as great as the standard, hence it has a focal length of y2 meter; and one of 10 diopters has a focal length of i/io meter and so on
tor lenses having a strength less than the standard of i meter the focal length will also be inversely as the power, and hence a Y* diopter lens will have a focus of 2 meters and a i/ioth diopter lens has a focus of 10 meters. In general, the less the dioptry or strength the longer is the focus, and the greater the dioptry or strength the shorter is the principal focus.
Convex lenses with a real principal focus arc indicated by the plus sign ( + ).