94 BACTERIAL DISEASES OF PLANTS 



rubber, the holes in which are smaller than any test tube but 

 are slit in four places 90 apart so as to receive and hold tubes 

 of any size. For very large tubes a second carrier is provided. 

 With these additions the apparatus is most convenient since 

 it can be rotated toward any part of the compass, tilted and 

 locked at any angle from vertical to horizontal, lengthened 

 or shortened as required, and the various parts shifted so that 

 the object to be photographed can be centered on the ground 

 glass with a minimum of labor. Two additional changes 

 w y ould make it nearly perfect (1) a cut in the top of the frame- 

 work at a given point (x) so that the object carrier might be 

 removed (lifted out bodily) without sliding it all the way to the 

 end of the frame- w r ork, where the background carrier is often 

 in place and must now also be removed to let it out; (2) the 

 object carrier can be moved to the right or left any distance 

 desired, but only 3 inches away from the central axis of the 

 stand, whereas sometimes it is desirable to move it farther, that 

 is, 4, 5, or 6 inches. This could be accomplished very easily 

 by lengthening the stop groove in the brass strap on the under 

 surface of the frame. 



Photomicrographic Work. Nearly all of the photomicro- 

 graphs used in this book were made on the small Zeiss upright 

 camera (Fig. 55). (For more details see "Bacteria in Relation 

 to Plant Diseases," Vol. I, and various text-books.) Dr. L. B. 

 Wilson of the Mayo Laboratories, Rochester, Minn., has de- 

 vised and described a very convenient upright stand. For an 

 illustration of this photomicrographic camera see the 1917 cata- 

 logue of the Spencer Lens Co., Buffalo, N. Y., p. 120. 



Light Filters. For the large horizontal photomicrographic 

 camera I have generally used the green, fluid, ray-filter known 

 as the Zettnow filter, but for planars the dry Wratten filters 

 made by the Eastman Kodak Company, Rochester, N. Y., 

 are very useful. (Catalogue and explanations for use will be 

 furnished by the Eastman Company on application.) 



Dry Plates and Developers. All these have been greatly 

 improved in recent years. The essentials in dry plates and films 

 are to have some giving strong contrasts and others correct 

 color values. It is sometimes impossible to tell in advance 



