208 Dr. G. J. Burch. [ Mar. 7, 
terms of its intensity when the lamp was at the greatest distance from the 
paper reflector, namely, 2 metres. “Ratio” is the quotient of the intensity 
of the blue-violet band by that of the red, and shows the proportion of blue-. 
violet to red necessary to produce an equal sensation under the conditions of 
the experiment, taking as unity the ratio for the minimum visible to the 
rested eye. 
_ Although the positions in which the lamp was placed for the second set of 
readings were the same as for the first set, the intensities are different, because 
in adjusting the two colours to equal luminosity by the Nicol prism, the one 
is increased and the other diminished. This is one of the drawbacks of the 
polarisation type of photometer. 
The third measurement is interesting. 
Moving my head towards the instrument in the dark, the cap of the eye- 
piece came sharply in contact with my face close to the eye. The bands, 
which had appeared red and blue, though barely visible, were instantly 
obscured by a luminous fog of after-effect, so that I could not take the ETE 
for some 30 seconds, and when I did, they both appeared colourless. . 
_ EHaperiment 8.—In this. case I remained more than two hours in total 
darkness, but did not take.the first record till nine minutes had elapsed.. The 
unit of intensity is the minimum visible at the end of two hours, 2.¢., with the 
lamp 3 metres from the paper. The slit was, I believe, narrower than in-the 
preceding experiment. 
During the Process of Resting in Darkness. 
Intensity of | Intensity of 



Time. red. blue-violet. Ratio. 
9 16°63 254-76 Peo? 
21 llr 57°61 clk 7 
25) 4°61 1282 2°78 
— 60 2-40 5 39 24 
120 1:00 1:00 1-00 

After Resting Two Hours in Darkness. - 

120 | 1-00 1:00 00 
122 1-20 1°63 1°34 
123-30 167 2°82 1°65 
125 2°69 5°14 USM 
127 5°04 12°44 2.°47 
128-30 13°21 55°82 4°23 
130 50:02 225°72 4°51 

