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Z. LEWIN,
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New York UniversiW, New York 3, N. Y.
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from the latter enter the eye at such angles that the refraction produced by the latter's lens system causes an image to be formed on the retinal surface a t the rear of the eye. The essential part played by the eye lens in determining the apparent size of the image and its position in space
lluU are c?f practtkd impwtaltce to those whu use, m teach Me use of, vnn&m itlJlnrmenlatirm and instmmentat technipues.
XXIV. Instrumentation For Micrometry and Microscopy-
Part Three
Figure 35. if on obiect is located a t the fosol plane of o converging lens, the diverging rays from eoch point of the object are refracted into bundles of parallel rays.
Z. Lewin. De~ortmentof Chemistry, New York University, New York 3; N. 'Y. 5.
Optical Principles of the Compound Microscope The purpose of the compound microscope is to produce a bright, clear, undistorted, and greatly enlarged image of the object under study. It has been shown in the preceding that in the case of individual lenses these requirements are mutually incompatible. For the image to he bright and clear, the lens must gather a. large amount of light from the object, i.e., it must have a large diameter, or aperture. But a large aperture inevitably results in distortion, and the distortion incremes rapidly as the power of the lens is raised. The special role of the compound microscope in providing a way around this impasse arises from the fact that the properties of a system composed of two lenses separated by a considerable distance (i.e., a distance equal to, or greater than the sum of the focal lengths) are strikingly different from those evidenced by these Lenses when close together. For thin lenses that are in contact, or dose together, the power ( = dioptry; D = l / j ) of the combination is the algehrrtic sum of the powers of the individual components. Thus, as illustrated in Figure 34, a converging lens of + 3 diopters, in oontact with a diverging lens of - 3 diopters combine to yield a system of 0 diopters. However, if the centers of these lenses are separated by a distance equal to the sum of their focd lengths (in this case, 66.6 cm) plus an optical inlmual, A, this combination can be shown to have an effective focd length, fro+, given by:
D,=+3
D2=-3
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Figure 34. A. The powers of lenrer in contact with each other are additive. Thus, a converging lens of +3 diopkrr placed close to o diverging lens of -3 dioptors giver a resultant power of zero. B. If the lenrer ore separated b y a distance larger than their focol lengths. the resultant effect 1% quite different. The image formed by the flrrt lens serves ar the obiect for the second lens. The power of the combination depends upon the optical interval, h
value, the resultant power will be +6 diopters. [ I n the standard compound microscope, the optical interval, A, is generally between 16 and 18 cm.]
Role of the Eye I n the example shown in Figure 34, if A = 33.3 em, then jComa. = 33.3, and D,.+ = +3 diopters. If,e.g., the optical interval is reduced to one-half of the preceding
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