182 METHODS IN PLANT HISTOLOGY 



hardener to harden the emulsion and an acid to counteract the effect 

 of the alkahne developer is used to dissolve the silver which has not 

 been affected by light. 



Acetic acid is often used as a "short-stop" between the developer 

 and the "hypo." 



Metol.—Metol should be completely dissolved in warm water before 

 adding any sulphite. It is a vigorous developer, bringing the entire 

 image up so quickly on the surface of the plate or film that the be- 

 ginner might stop development too soon. Turn the plate over and 

 look at the other side to make sure that development has taken place 

 throughout the entire thickness of the film. Potassium bromide re- 

 strains the rapidity of the action, allowing the metol to soak through 

 the emulsion before the surface development is too extreme. It is al- 

 most always used in combination with hydrochinon. Elon, pictol, and 

 rhodinol are about the same as metol and may be substituted for it in 

 formulas. 



Hydrochinon. — This developer comes in needle-like crystals which 

 can be dissolved in water at room temperature. If the temperature is 

 below 50° F., it does not act. It develops best at temperatures from 

 65° F. to 70° F. It is not used alone except for maps, graphs, and such 

 things, where dead black lines and dots with a clear glass background 

 are desired. It is generally used in combination with metol, forming 

 the famous M.Q. developer, in which the metol softens the harshness 

 and gives detail even in the shadows, so that the result is an artistic 

 picture, whether you want a print on paper or a lantern slide. A much 

 larger proportion of hydrochinon is needed for paper prints than for 

 plates or films. 



P7/ro.— This is often called pyrogallol or pyrogalhc acid. Profession- 

 al photographers generally use pyro. Combined with metol it is widely 

 used in tank development of films. Properly used, it produces splendid 

 negatives; but it has been replaced to a great extent, by newer devel- 

 opers. 



Sodium sulphite.— In a tight can or a well-stoppered bottle, this 

 reagent will keep for years. It takes no part in the developing but acts 

 as a preservative, its function being to prevent too rapid oxidation of 

 the metol or other developing agent. Weigh carefully, because too 

 much or too little may cause fog, especially with hydrochinon. 



Potassium metahisulphite.— This acid salt is probably the best pre- 

 servative when pyro is the developing agent, for pyro keeps only in an 



