92 BACTERIAL DISEASES OF PLANTS 



The Zeiss Planar lenses, Series la, may have a word here. 

 They are fully described in the Zeiss catalogues and I have 

 figured Nos. 1 to 5 (those best adapted to photomicrographic 

 work) in "Bacteria in Relation to Plant Diseases," Vol. I, p. 132, 

 together with the special substage condensers to be used with 

 them. These lenses have a focus varying from 20 mm. to 100 

 mm. and yield, when stopped-down, images ranging from 1 to 

 5 inches in diameter. They may be used either on the micro- 

 scope or on the ordinary camera. They are very rapid lenses 

 working with a minimum of light. They yield very sharp 

 images, but have not much depth of focus. As stated in the 

 catalogue ''Focusing must be done with the most scrupulous 

 care." I generally use a hand lens, focusing on a clear-glass 

 image, and stop-down as much as possible to get all the depth 

 of focus possible. In selecting a focus with the stop wide open 

 it is well to remember that the increased depth of sharpness 

 to be obtained later by stopping-down will be mostly away 

 from the observer rather than toward him. 



Camera Stands. — For many purposes a simple tripod, or a 

 home-made device such as that figured in "Bacteria in Relation 

 to Plant Diseases," Vol. I, p. 133, is sufficient, but for use with 

 planars and generally for best results in lighting, we now use 

 a universal position stand, i.e., the stand made by Folmer 

 and Schwing (Eastman's catalogue of photographic apparatus 

 and materials, p. 50) with important modifications by James 

 F. Brewer, as shown in Figs. 59-60. These improvements 

 consist of (1) a device at the bottom of the stand for rotation; 

 (2) castors for translocation; (3) a swing device under the back- 

 board for raising or lowering the camera as a whole and locking 

 it in the position desired; (4) an object carrier movable not only 

 backward and forward but also to either side and away from 

 the central axis of the stand; (5) a carrier behind the preceding 

 for the background; (6) a screw device for shortening or lengthen- 

 ing the distance of the camera as a whole from the object; 

 (7) a test-tube rack attached to the front part of the object 

 carrier and sliding up and down. In this carrier are a series 

 of inch holes through which are thrust the tubes to be photo- 

 graphed, the same being held in place by a lining of perforated 



