Perhaps the most important tests needed are in regard to the sensitiveness of the plates.  Most plate makers use the wet plates as their standard, giving the sensitiveness of the dry plates at from two to sixty times greater; but as wet plates vary quite as much as dry ones, depending on the collodion, condition of the bath, etc., this system is very unsatisfactory.  Another method, employed largely in England, depends on the use of the Warnerke sensitometer.  In this instrument the light from a tablet coated with luminous paint just after being exposed to a magnesium light is permitted to shine through a colored transparent film of graduated density upon the plate to be tested.  Each degree on the film has a number, and, after a given exposure, the last number photographed on the plate represents the sensitiveness on an empirical scale.  There are two or three objections to this instrument.  In the first place, the light-giving power of the luminous tablet is liable to variations, and, if left in a warm, moist place, it rapidly deteriorates.  Again, it has been shown by Captain Abney that plates sensitized by iodides, bromides, and chlorides, which may be equally sensitive to white light, are not equally affected by the light emitted by the paint; the bromides being the most rapidly darkened, the chlorides next, and the iodides least of all.  The instrument is therefore applicable only to testing plates sensitized with the same salts.

In this investigation it was first shown that the plates most sensitive for one colored light were not necessarily the most so for light of another color.  Therefore it was evident that the sun must be used as the ultimate source of light, and it was concluded to employ the light reflected from the sky near the zenith as the direct source.  But as this would vary in brilliancy from day to day, it was necessary to use some method which would avoid the employment of an absolute standard of light.  It is evident that we may escape the use of this troublesome standard, if we can obtain some material which has a perfectly uniform sensitiveness; for we may then state the sensitiveness of our plates in terms of this substance, regardless of the brilliancy of our source.  The first material tried was white filter paper, salted and sensitized in a standard solution of silver nitrate.  This was afterward replaced by powdered silver chloride, chemically pure, which was found to be much more sensitive than that made from the commercial chemicals.  This powder is spread out in a thin layer, in a long paper cell, on a strip of glass.  The cell measures one centimeter broad by ten in length.  Over this is laid a sheet of tissue paper, and above that a narrow strip of black paper, so arranged so as to cover the chloride for its full length and half its breadth.  These two pieces of paper are pasted on to the under side of a narrow strip of glass which is placed on top of the paper cell.  The apparatus in which the exposures are made consists of a box a