. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. 102 PLANT PHYSIOLOGY retioal value of p is only slightly above the lower limit of microscopic visibility. An objective of numerical aperture , with the 365 mu ultra- violet spectral line (of mercury) will give 19 per cent, more resolving power than the best microscopic system with white light. But there are difficulties Glass IS practically opaque to light waves of less than 300 mp, , to ultra- violet light; furthermore, such light is invisible to the
. Contributions from the Botanical Laboratory and the Morris Arboretum of the University of Pennsylvania, vol. 14. Botany; Botany. 102 PLANT PHYSIOLOGY retioal value of p is only slightly above the lower limit of microscopic visibility. An objective of numerical aperture , with the 365 mu ultra- violet spectral line (of mercury) will give 19 per cent, more resolving power than the best microscopic system with white light. But there are difficulties Glass IS practically opaque to light waves of less than 300 mp, , to ultra- violet light; furthermore, such light is invisible to the human eye The first difBculty is overcome by using quartz lenses, and the second by sub- stituting the photographic plate for the human eye. The ultraviolet microscope was presented to the biological world some thirty years ago by Kohler of the Zeiss scientific staff at Jena. The instru- ment has a potential resolving power twice that of the best optical systems using visible light, which means that it gives twice as much detail as do the best lenses with ordinary light. It also gives greater optical differentiation ^.e greater contrast, for example, between the translucent, glass-like parts ot a cell. It thus obviates the need of staining protoplasm, which is diffi- cult of accomplishment in living matter. The method was originaUy in- tended for metallographic studies, but in spite of a promising future, little ontstanding work was done with the ultraviolet microscope. Lucas (14) of the Bell Telephone Laboratories in New York, has revived its use. Ultraviolet light of a known wave length is obtained by passing light from a suitable source, such as a spark between cadmium or magnesium electrodes, through a quartz prism which resolves or breaks it up into its component wave lengths. Ultraviolet rays of about 275 mji wave lengths are selected. As these are invisible, the optical image cannot be seen in the ordinary way. It must either be photographed or visualized on a fluores- c
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