124 BACTERIAL DISEASES OF PLANTS 



to allow the light to penetrate the thickest parts; (7) no specific 

 rules can be given as to length of exposure. It varies enor- 

 mously with the subject, objective, eye-piece, bellows length, 

 size of diaphragm, sensitiveness of dry plate, kind of screen and 

 source of light, and if sunshine is use.d, state of the sky, latitude, 

 time of day and time of year. In Washington, using planar 

 lenses with short bellows length and bright sun light, it is gen- 

 erally only fractions of a second (sometimes as short as i-f oo 

 second). With high powers, long bellows, small stop and dark 

 subjects it is often five minutes or more. Sometimes it is very 

 much longer if the source of light is feeble. Using the light of 

 the open sky, Cramer's Iso slow plates, and a densely stained 

 slide, I have sometimes exposed the plate for an hour, but this, 

 of course, is exceptional, although under such conditions 10 to 

 20 or even 30 minute exposures are common for magnifications 

 of X 1000. Toward sunset the actinic effect of the light falls off 

 very rapidly and the exposure must be correspondingly lengthened, 

 30 minutes to an hour bemg not unusual. 



If the object is a small one in the center of an otherwise 

 well-lighted field, the exposure should be much shorter, naturally, 

 than when there are deeply stained objects in many parts of the 

 field. 



The beginner will do well to confine himself to the simple 

 upright camera. If he avoids direct sunlight, focusing on the 

 diffuse light of the sky, he will have more leeway in his exposures 

 and will not require a shutter. The exposures with such an 

 apparatus are very much lengthened, even on clear days, as 

 compared with arc light or heliostat light, but this is no serious 

 difficulty. With such an apparatus, using Cramer's Iso-chro- 

 matic slow plates, a Zeiss 2-mm. apochromatic oil-immersion 

 lens, a No. 4 compensating ocular, and a magnification of X 1000, 

 the time of exposure here in Washington on a clear summer day 

 between 9 a.m. and 3 p.m. varies, with the nature of the section, 

 and the stop, from one minute to 20 minutes or more; with all 

 else the same, but using a Zeiss 16-mm. apochromatic objec- 

 tive, the time varies usually from five seconds to 10 minutes ac- 

 cording to the subject and the stop. A little experience will 

 enable the painstaking student to obtain good pictures, especially 



