210 Lieut. L. Darwin on the Rate of the Decrease of 



reached the eye, and the light of the room did not reach the 

 surface of paint. Besides these reasons for observing through 

 a small hole, it is better to do so, because the light given off 

 by the illuminated tissue seems to vary with the angle at 

 which it is observed, and the readings would not be constant 

 if the eye were not in a fixed position. Experiment showed 

 that the diffused light in the box from the illuminated tissue 

 paper was not enough to have any effect on the phospho- 

 rescent paint. Both the paint and the tissue were observed 

 through a solution of cupric sulphate, and blue glass ; this 

 did not, as far as I could see, cause any alteration in the 

 colour of the paint. If the blue glass alone was used, the tissue 

 looked pink in comparison with the paint ; and with the cupric 

 sulphate solution alone it looked green ; but with the two 

 the colour was so nearly imitated, that when the intensity of 

 the light was the same I could only distinguish the two 

 surfaces by their positions. 



The illumination of the tissue was estimated by the distance 

 of the burner from the tissue ; this assumes that the light 

 given off by the tissue varies directly as the light striking it 

 on the reverse side. Bright sunlight was reflected onto the 

 the painted surface by a mirror; and after it had acted for a 

 few seconds, the room was darkened, with the exception of the 

 light of the standard burner. For five minutes before this time, 

 I took the precaution of hiding my eyes, so that no light could 

 get at them. As soon after this as possible, observations were 

 taken. Two slightly different methods were employed. At 

 first the burner was placed at a definite distance from the 

 tissue, and the time was noted at which the paint and tissue 

 first appeared to be equally bright, and also that at which 

 they first appeared to differ again in illumination ; the mean 

 of these two times was taken as the time of equal illumination. 

 The burner was then moved away to a further fixed position, and 

 another observation taken in the same manner. After five 

 observations had been taken in this way, the rate of decrease 

 of the light became very slow, and this method did not work 

 well. The distance of the burner to obtain equal illumination 

 was then found by moving it backwards and forwards, and 

 noting the position at which the illumination of the two sur- 

 faces appeared the same ; this was done four times as rapidly 

 as possible, two before and two after the time at which an 

 observation was wanted. The following table gives the mean 

 of two series taken in this manner. The first column gives 

 the illumination of the paint : the light given off by the tissue 

 when the burner was nine inches from it is taken arbitrarily 

 as 100; and the other illuminations are calculated accordingly 



