PRACTICAL PHOTO-MICEOGBAPHY. 93 



plate of the same batch would do so. If with any of our 

 "Normal Developers" no image at all is visible after 

 thirty seconds, the plate may be put down as underex- 

 posed. The image appears in the following order of rapidity 

 with our three Normal Developers : 1st Pyro- Ammonia ; 2nd 

 Pyro-Carbonate ; 3d Ferrous Oxalate. This is to be taken as 

 a general rule in comparing the Carbonate with the Oxalate 

 developer. Bromide always slows the appearance, and also 

 the acquisition of density and detail, of the image and free 

 bromide in the developer has a much greater apparent effect 

 on the carbonate and ferrous developers than on the ammonia 

 developer. Half a grain of bromide in the carbonate or fer- 

 rous developer has at least as much retarding effect as a grain 

 in the ammonia developer. Bromide restrains both detail and 

 density, but luckily it restrains detail more than density. The 

 chief use of free bromide in the developer is to give us time 

 to watch progress and to stop progress at the proper moment, 

 but bromide is absolutely necessary to prevent fog with certain 

 plates used with the ammonia developer. In the case of the 

 carbonates the carbonic acid evolved acts as a restrainer and 

 retarder ; in the case of the ferrous oxalate, ferric bromide is 

 formed in considerable quantity, and restrains and retards de- 

 velopment. Hence a ferrous oxalate developer used over and 

 over again works each time more slowly and more feebly than 

 the time before. We deprecate the repeated use of the same* 

 dose of pyro-developer ; but repeated use of one dose of ferrous 

 oxalate is quite permissible up to a point which will easily be 

 known by muddiness of the solution, and slowness of its action. 

 If the image starts about 10 or 15 seconds after the devel- 

 oper is applied to the plate, we must be prepared for over- 

 exposure of the high lights at least. With subjects presenting 

 violent contrasts of density or color-actinism, we are almost 

 bound to over-expose the high lights ; but with ordinary sub- 

 jects the image, if it appears at all in 15 seconds, should show 

 only the highest lights at that stage ; and the half tones of a 

 properly exposed plate will follow the high lights without 

 any lagging or apparent reluctance. In fact, the develop- 

 ment should proceed steadily, without stoppage and without 

 precipitancy from start to finish, so far as detail is concerned. 



