252 Captain W. de W. Abney. Effects of [Mar. 18, 



they were more marked when the salts were precipitated separately. 

 The only method at that time available to distinguish the maxima 

 was by the appearance of a negative photograph of the spectrum 

 impressed upon it, and hence there was a liability to be deceived, 

 since densities in deposit which are nearly alike are apt to be over 

 looked. 



I utilised my method (" Phil. Mag.," 1885) of obtaining patches of 

 monochromatic light from the spectrum, in examining afresh dif- 

 ferent salts of silver as regards sensitiveness to different rays. The 

 experiments were conducted in the following manner: — A sensi- 

 tometer, designed by Mr. Spurge, was brought into use ("Photo- 

 graphic Journal," 1882, vol. vi). This consists of a series of small 

 chambers, about 1 cm. square in section, and 2 cm. deep. Below 

 these chambers is a sheet of brass, punctured as shown in the 

 figure, each such puncture corresponding with the square chamber. 



Numbers are also punctured in the brass triangle, to correspond to 

 the order of intensity in which the light is admitted to each chamber. 

 Below this brass plate can be placed a sensitive plate to be tested. 

 The tops of the chambers are also closed by a brass plate, in which 

 holes of different diameters are punctured. The area of each hole is 

 V2 that of the next, and the total number of chambers is 30. It 

 will be thus seen that the difference in light from an equally illu- 

 minated surface admitted to the first and last holes is immense. 



To obtain a surface equally illuminated two sheets of finely ground 

 glass were used, one placed about one-eighth of an inch from the holes, 

 and the other about a centimetre away from the first. It was found 



