136 BACTERIA IN RELATION TO PLANT DISEASES. 



may also be obtained by photographing the tubes against a north light inside a 

 box with blackened walls. The box may be 8 X 8 X 12 inches, open at each end, 

 and painted inside with a mixture of lamp-black and turpentine. One open end is 

 pointed to the window, the other to the lens of the camera. In the middle of the 

 box, crosswise, on top, a row of ^-inch holes (i*4 inches apart) is bored, and the 

 tubes to be photographed are thrust through these. If images of outside objects 

 appear on the ground glass, they may be cut out by pasting white tissue-paper on 

 the window-glass (this should be glued only at the corners). 



The kind of plate used depends upon the object. If the contrasts are very great 

 e.g., a waterfall or bright rock surrounded by vegetation, or an interior with the 

 camera pointed toward windows a double-coated non-halation plate should be used 

 (Hammer's Aurora plates are very good ; plate 6 was made with such a plate). In the 

 absence of such plates most of the halation may be avoided by squeegeeing to the 

 back of the dry plate, before loading, a black paper soaked in glycerin (plate 14 was 

 made in this way). The dry plate should be placed face down on dry blotting paper 

 during this process, and, of course, the glycerin-soaked paper must be cut in advance 

 to fit the dry plate. Much may be done during development to avoid violent con- 

 trast if one knows how, the quantity of pyrogallol or ortol being greatly reduced and 

 the development prolonged. This gives a thin negative full of detail. 



For many purposes isochromatic plates are invaluable ; for other purposes ordi- 

 nary plates will give better results. Which kind is best adapted to a particular 

 subject will depend on what is wanted. In general, for this sort of work, the full 

 contrast of the original is desired, and the kind of plate which will give it is best. 

 Even some exaggeration of the contrasts of the original is not objectionable in many 

 cases if the prints are to be used for half-tone reproductions, since contrast is often 

 reduced in the half-tone process, and there must be exaggeration in the photograph 

 if the half-tone picture is to correctly represent the object. An example or two will 

 help the beginner to judge. If we have black spots on a green background and 

 use a rapid non-isochromatic plate, we will get the result shown in fig. 126, which 

 may be contrasted with fig. 127, made from the same leaf, but on an isochromatic 

 plate. For such cases the isochromatic plate is of course the one to be selected. In 

 the same way, if one desired to reproduce the variegations of a pansy with the full 

 value of each color, he would use an isochromatic plate. On the contrary, white 

 spots or stripes on a green leaf, or yellow colonies on an agar or gelatin plate, or red 

 spots on a white ground, will stand out better if the photograph is made on a Seed's 

 2/-X non-isochromatic plate or its equivalent. Red spots on a green background 

 require an isochromatic plate. Black spots on a yellow or orange ground usually 

 require for good contrasts an isochromatic plate. Some yellows, however, take pale, 

 while others take dark. 



In making photomicrographs little trouble is experienced with low powers, but 

 there is considerable difficulty in making good negatives of bacteria in tissues, using 

 high powers. A few hints may be of service. With upright stands and certain 

 objectives the beginner frequently has difficulty in securing a uniformly lighted 

 field. This trouble may be obviated by throwing the light from the mirror not 



