DEVELOPING THE IMAGE. 221 



produced is in general sufficient to produce negatives by 

 the wet process up to 2500 diameters with three minutes' 

 exposure. Fig. 112 shows that difficultly resolvable diatom 

 the A.pellucida, under an amplification of 960 diameters. 



After the sensitive plate has been exposed the necessary 

 length of time by either of the foregoing methods of illu- 

 mination, it is ready for development. It is, therefore, 

 taken into a room from which every trace of actinic light 

 has been excluded, and there the picture is brought to 

 view by the use of the developing solution. Nearly every 

 photographic manipulator has his own way of developing, 

 though the general principles are the same, but that given 

 by Messrs. Wratten and Wainwright for their instantaneous 

 dry plates will perhaps be found the most suitable for 

 photo-micrographical purposes. 



STOCK SOLUTION A*. 



Ammonia liquor, fort. I oz. 



Potass bromide 60 grains 



Water 2 oz. 



DEVELOPER. 



Pyrogallic acid . . 6 grains 



Stock Solution A* 20 drops 



Water 2 oz. 



Lay the exposed plate in a dish of cold water to soak 

 while the pyrogallic acid is mixed. For each i or 5 x 4 

 plate use 6 grains of pyro., diluted with 2 oz. of water. 

 First pour off the water from the plate and apply the 

 pyro. solution, then add five drops of stock solution A*, 

 and keep this weak developer on the plate until the highest 

 lights are pretty well visible ; then add from 1 5 to 20 drops 

 more of A* to finish development. By this method more 

 of the film is employed and greater density is obtained. 

 Whenever any of solution A* is to be added to the pyro. 

 solution it should be first dropped into the developing cup, 



