. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. 100 PLANT PHYSIOLOGY. Fig. 1. The microscope of Robert Hooke (1660) AJS. greater resolving power that is needed. Resolving power is the capacity of a lens to separate one line or point from another lying very close to it. Let us take a specific case: A diffraction grating, as used for the formation of spectra, may contain 1000 lines to a millimeter. Microscope lenses of the highest power easily distinguish these as individual lines clearly separated from each other
. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. 100 PLANT PHYSIOLOGY. Fig. 1. The microscope of Robert Hooke (1660) AJS. greater resolving power that is needed. Resolving power is the capacity of a lens to separate one line or point from another lying very close to it. Let us take a specific case: A diffraction grating, as used for the formation of spectra, may contain 1000 lines to a millimeter. Microscope lenses of the highest power easily distinguish these as individual lines clearly separated from each other. But ,f there are two lines where there was but one before, then each line will be separated from its neighbor by p. It would, in this case, be barely possible to distinguish individual lines if the microscope lenses and the eye of the observer were of the best. We can go no further with direct vision. As a result, efforts to increase the powers of the micro- scope have of late been directed toward methods of illumination. Illumination wTT^i/""^ °^f ^ V r ^^' P'""""" °^ microscopic illumination have been followed; either light of other wave lengths is used, or white light is applied in differen ways. The light, other than composite white light so far most successfully used is ultraviolet. The ultraviolet microscope «nZt' "'1^''"^ """"^ °' ^ '""' '^ proportional to its numerical aperture and inversely proportional to the wave length of the light used. In other '/ ^l^- SEIFRIZ: PHYSICAL METHODS OF RESEARCH ON PROTOPLASM 101 Tr^L*^! ^'â ^^*^'* *^^ numerical aperture and the shorter the wave length o± light, the smaller the structure which can be imaged by the lens. Written m the form of an equation: Smallest structure .^Vhl^-^^^^ length of light numerical aperture The average length of a wave of white light is p (5500 A); accordingly, a lens with a numerical aperture of will r
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