150 PRACTICAL PHOTOMICROGRAPHY. 



MR. A. COWAN'S FORMULAE FOR DEVELOPMENT OF GELATINE 

 CHLORIDE PLATES. 



a. Ferrous protosulphate 140 grains 



Sulphuric acid 1 minim 



Water 1 ounce 



Add one part of a to three parts of one of the following. 



FOR COLD TONES AFTER SHORT EXPOSURE. 



b. Potassic citrate 136 grains 



Potassic oxalate 44 grains 



Hot water 1 ounce 



FOR WARM TONES AFTER LONG EXPOSURE. 



c . Citric acid 180 grains 



Ammonic carbonate 60 grains 



Water 1 ounce 



MR. B. J. EDWARDS' FORMULA. 



a. Neutral potassic oxalate 2 ounces 



Ammonic chloride 40 grains 



Distilled water 20 ounces 



b. Ferrous protosulphate 4 drams 



Citric acid 2 drams 



Potash alum 2 drams 



Water distilled 20 ounces 



For developing mix a and 1) in equal parts. 



The developer is allowed to act until the image is seen 

 plucky in every part, the plate is then well washed and fixed 

 in " hypo," of usual strength. If any scum, due to salts in 

 the water, is found, it may be removed by the acid-alum solu- 

 tion given on page 63, the solution being poured on to the plate. 



Beautifully clear slides ought to result from this treatment 

 of gelatine chloride plates. These plates may be made much 

 more sensitive than those usually sold, but, as a rule, the more 

 sensitive plates are inferior in quality. 



In developing these plates, it is well to develop to full den- 

 sity at least, rather beyond than short of it. It is easy and 

 often advantageous to reduce the slides after fixation thus : 

 (Mr. Cowan.) 



" Strong solution of iron perchloride % ounce 



Hydrochloric acid % ounce 



Water.. ..20 ounces 



