I40 PRACTICAL PHOTO-MICROGRAPHY. 
another in this respect. An average negative may 
require about fifteen seconds’ exposure at eighteen 
inches from a gas burner with the Kodak permanent 
paper, a dense negative will yield good results after 
thirty seconds; some negatives need even more 
than this. If a negative is so thin as to stand no 
more than five or six seconds, it is too thin to yield 
really good results unless under exceptional circum¬ 
stances. 
There is no better developer for bromide paper than 
the ferrous oxalate. We make solutions of neutral 
potassium oxalate and iron protosulphate, saturated in 
water at 60 degs. Fahr. To make the iron solution we 
first acidify the water distinctly with sulphuric acid— 
about half-dram acid to a pint of water—and then we 
add the iron salt, shaking or stirring well occasionally. 
The developer consists of one part of the iron solution 
to five or six parts of the oxalate, and to each ounce 
of this mixture it is advisable to add half a grain of 
potass, bromide. The separate solutions keep for a 
long time, but the mixed do not keep long; in mixing 
the two the iron should be poured into the oxalate , and not 
vice versa. 
Note. —The combination of iron sulphate with potass, 
oxalate produces ferrous oxalate, which is soluble in 
excess of potass, oxalate. 
One part of iron to eight of oxalate answers well for 
the Kodak paper, but if the blacks are inclined to blue 
one part to six may be used. Thinness of negative 
may be met by keeping down the exposure and in- 
