16.--Citric acid. 90 grains.
Carbonate of magnesia. 57 "
Citrate of potash. 54 "
Oxlate of potash. 18 "

17.--Citric acid. 72 grains.
Carbonate of magnesia. 45 "
Citrate of potash. 54 "
Oxalate of potash. 18 "

18.--Citric acid. 60 grains.
Carbonate of magnesia. 38 "
Citrate of potash. 68 "
Oxalate of potash. 22 "

_A more Intense Blue Black._
19.--Citric acid. 30 grains.
Carbonate of magnesia. 18 "
Citrate of potash. 100 "
Oxalate of potash. 33 "

_A Clearer Blue._
20.--Citrate of potash. 136 grains.
Oxalate of potash. 44 "

In the photographic exhibition at Florence, the firm of Corvan[1]
places on view a frame containing twenty proofs produced by the
foregoing twenty formulæ, in such a way that the observer can compare
the value of each tone and select that which pleases him best.--_Le