160 PROCEEDINGS OF THE AMERICAN ACADEMY 



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 liglit of an- 

 other 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 stan- 

 dard of light. It is eviilent that we may escape the use of this trou- 

 blesome 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 jjaper 

 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 as to cover the chlo- 

 ride 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 little over a meter in length, closed at the 

 top by a board, in which is a circular aperture 15.8 cm. in diameter. 

 Over this board may be placed a cover, in the centre of which is a 

 hole .05 cm. in diameter, which tlierefore lets through .00001 as much 

 light as the full aperture. The silver chloride is placed at a distance 

 of just one meter from the larger aperture, and over it is placed the 

 photographic scale ; which might be made of tinted gelatines, or, as in 

 the present case, constructed of long strips of tissue paper, of varying 

 widths, and arranged like a flight of steps ; so that the light passing 

 through one side of the scale traverses nine strips of paper, while that 

 througli the other side traverses only one strip. P^ach strip cuts off 

 about one sixth of the light passing through it, so that, taking the mid- 

 dle strip as unity, the strips on either side taken in order will transmit 

 approximately, — 



