156 
C APT Am ABNEY ON THE SENSITIVENESS 
their use had to be abandoned. Reduction of intensity by means of a polarizing 
apparatus had already been tried and found unsatisfactory, when the beam of light 
approached the point of absolute visual extinction. 
2. Gelatine Wedge in Ammlar Form. 
It occurred to me that a few years ago Mr. Leon Warneeke had brought out a 
“ sensltometer ” (an instrument for measuring the sensitiveness of a photographic 
plate) in which the apparatus for reducing the intensity of light admitted to a 
sensitive surface consisted of an annulus of gelatine of gradually increasing thickness, 
coloured either by a dye or by a powder of any colour which might be desired, I 
applied to Mr. Warnerke to prepare for me some of these sensltometer screens, and 
to use as a powder very finely divided lampblack. He kindly gave me several 
specimens of three varieties, one having but little pigment mixed with the gelatine, 
another rather more, and the third with strong absorption. This last reduced the light 
which penetrated the thinnest part to part when it had to pass through the 
thickest portion, the second only reduced it to rather less than ycnfo part, and for my 
purpose I selected this last with which to work as a rule, though in some cases the 
more opaque annulus was used. I am informed that the mode of the preparation of 
the annulus is as follows In a perfectly flat and thick steel disc a groove of 
gradually increasing depth is cut by a proper machine into an annulus about 1 inch 
broad. When the depth of the groove has been tested and found to increase 
proportionally to the arc of the circle, replicas of the disc and groove are made in 
non-oxidizable metal. The finest pigment is then mixed with semi-liquid gelatine, and 
when thoroughly incorporated the viscous liquid is poured into the groove, the top 
surface of the disc being very carefully levelled. A sheet of worked glass is then laid 
on the surface of the disc, and any excess squeezed out except a very minute film 
which appears colourless, and for which there is no means of escape. When the 
gelatine has properly set, the glass plate is removed with the relief of the groove 
attached to it. The gelatine annulus is allowed to dry, and is then ready for use. 
It struck me that such an annulus might be substituted for glass wedges, and after 
selecting one which %vas suitable as to colour absorption and regularity of absorption 
coefficient (if one may so call it), I finally determined to adopt it. It may be said 
here that not every specimen gave equally good results. It is only when the glass 
plate is perfectly flat that regularity in the increase in thickness, and consequent 
uniformity of coefficient per unit angle can be secured. The diminution in light in 
its passage through the annulus of pigmented gelatine is evidently due to a different 
cause to that of coloured glass. In the latter case it is due to true absorption, in the 
former to the obstruction by fine opaque particles. If the opaque particles possessed 
colour some of the light passing through the gelatine would be tinged with the light 
reflected from their surfaces, and if they were semi-transparent there would be not 
