44 The Microscope 



21. Each has its ardent supporters. Every photograph in this book is re- 

 produced from Kodak Plus X negatives. 



Positive Materials. Positive materials are also emulsions of gelatin and 

 silver halides coated on a support. This support may be either paper, to 

 produce a print, or glass, to produce a lantern slide. The criteria of posi- 

 tive materials are speed, contrast, and, in papers, surface. 



Speed. All positive materials are relatively slow. The slowest are "con- 

 tact" papers, coated with a silver chloride-silver bromide emulsion, which 

 are pressed into contact with the negative before exposure. Contact prints 

 are practically never used in scientific photography since projection print- 

 ing, which may or may not involve enlargement, gives far better control 

 of the process. 



Prints are therefore made by projecting an image of the negative onto 

 the face of bromide paper. There is little variation in speed between the 

 products of various manufacturers, and paper should be selected entirely 

 on the criterion of contrast. 



Contrast. The printing process is the right place to control the contrast 

 of the final picture. Every attempt should be made to prepare a negative 

 of medium density and medium contrast; from this, any kind of print can 

 be produced. Some kind of print can be made from an extremely con- 

 trasty negative on soft paper; a print of sorts can be made from a flat 

 negative on extremely harsh paper. Neither print will be as good as 

 one made from a medium negative on medium paper. Most manufac- 

 turers make papers in five grades numerically designated from No. 1 



Figs. 48, 49, and 50. The control of contrast by selection of printing papers. Fig. 49 

 shows a print of a normal negative on normal (No. 3) paper. Fig. 48 shows the same 

 negative printed on very soft (No. 1) paper and Fig. 50 on very contrasty (No. 5) 

 paper. The very contrasty print reproduces better, but the center print is better for 

 observation. 



