122 BACTERIAL DISEASES OF PLANTS 



wright's Panchromatic plates, which give nearly perfect color 

 values, and for this reason are so sensitive to red light that they 

 should be developed from start to finish in the dark, recommen- 

 dations to the contrary notwithstanding. Even with the best 

 plates it is sometimes impossible to get the contrast of nature 

 unless color screens are used. See Fig. 281. Here a bluish 

 black stripe on a pale green ground, perfectly distinct to the 

 naked eye, defied all attempts at photographing until the ex- 

 posure was made through a yellow screen. See also Fig. 239 of 

 cotton leaves and bracts and Fig. 47. 



As a developer, rodinol (sometimes called cytol), which is 

 ready for use on dilution with water, leaves little to be desired. 

 Many developers are poisonous and iritating to the skin. The 

 hands, therefore, should be kept out of them as much as possible 

 and always washed immediately after they have been dipped 

 into the tray. 



Prints for filing, or for use of the photo-engraver should be 

 on paper that does not curl. Films or papers that curl are 

 a source of endless vexation. We now employ two styles of 

 Eastman's Velox paper, viz., Special Glossy Velox paper for 

 hard (contrasty) negatives and Regular Glossy Velox paper for 

 soft negatives. In printing, dense negatives may be exposed to 

 daylight for a few seconds (10 to 20, at some distance from the 

 window), but thin negatives must be exposed to a weaker light. 



Reproductions from photographs are usually made on copper 

 by the half-tone process. Half-tones illustrative of patholog- 

 ical appearances (details) should be made with a screen not 

 coarser than 150 meshes to the inch, and in many subjects im- 

 portant features will be lost if the screen is not finer, i.e., 175 

 meshes to the inch. On good paper, with a good pressman (and 

 this is all important), there is no difficulty in getting a clear 

 impression from a well-made half-tone having 175 meshes per 

 inch, only the paper must be suitable, must be backed up properly 

 and printed carefully with just the proper amount of good ink. 

 Many of the smudges passing for illustrations in current publi- 

 cations are a disgrace both to the scientific man and to the printer. 

 I provide against loss of important small details through un- 

 avoidable defects in the half-tone, by enlargements to twice 



