. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. 132 BACTERIA IN RELATION TO PLANT DISEASES. flat surfaces he will select a lens with no great penetration, but with a very clear field, sharp to the edges, ?.<?., a Planar or some similar lens. If he needs a lens with very little depth of focus (but more than the Planar) and one allowing dark objects to be photographed in a very short time, e. g., luminous bacteria by their own light, he will select a Zeiss Unar or its equivalent, /. t\, an extremely rapid lens. If he desires in one picture as much as possible of a land
. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. 132 BACTERIA IN RELATION TO PLANT DISEASES. flat surfaces he will select a lens with no great penetration, but with a very clear field, sharp to the edges, ?.<?., a Planar or some similar lens. If he needs a lens with very little depth of focus (but more than the Planar) and one allowing dark objects to be photographed in a very short time, e. g., luminous bacteria by their own light, he will select a Zeiss Unar or its equivalent, /. t\, an extremely rapid lens. If he desires in one picture as much as possible of a landscape, ^ a large tree or an interior, he will select an extremely wide-angle lens rather than one distinguished for its rapidity or for the perfection of its definition, e. g.^ a Zeiss Protar, series V. The Double-Protar, series VII«, combines as wide an angle, as flat a field, as great rapidity, and as sharp a definition as it is possible, apparently, to obtain in a lens and at the same time have great depth of focus. These lenses may also be unscrewed and each half used separateh', if one wishes some portion of a picture more highh- magnified. They are furnished with front and back lenses of equal or unequal focal distance, as may be desired. In using Planars and all lenses which magnify, it is necessary to secure a very exact focus wiUi the stop wide open, for, unlike lenses which give pictures less than. Fig. 122.* actual size, only a very little increased depth of focus can be obtained by stopping down. With many objects—e. g., the surface of a leaf, or of bacterial colonies— there is considerable difl!iculty in deciding which is the proper focus when a Planar is used, what seemed like a good focus often yielding a poor negative. On this account the writer is in the habit of focusing on a fragment of very fine, sharp print laid on the surface of the leaf or of the agar-plate near the colonies to be photographed. A lens magnif)'ing 6 times is used in judging of the
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