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, /. e., 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, /'. c., an extremely rapid lens. If he 

 desires in one picture as much as possible of a landscape, e.g., 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 Vila, 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 separately, if one wishes some portion of a picture more highly 

 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 with the stop -vide 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 difficulty 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 011 the surface of the leaf or of the agar-plate near the colonies to be 

 photographed. A lens magnifying 6 times is used in judging of the image on the 

 ground glass, and when the best possible focus has been secured, the paper is removed, 

 the lens is stopped down two-thirds, and the photograph is made. In case of white 

 colonies the best results are obtained by resting the Petri dish on a piece of black 

 paper while the photograph is being made. The exposure is shortened by illumi- 

 nating the surface of the object with a bright beam from a mirror. The apparatus 



*Fic. 122. Zeiss Planar lenses, series la, Nos. I to 5. Nos. i, 2, and 3 may be attached to the 

 funnel-shaped carrier shown in the figure. This screws into the top of the microscope barrel in 

 place of the eye-piece tube. The one attached is No. 3. The condensing lenses necessary for these 

 Planars are also shown in this figure, at right and left. 



