COLORTMETRICAL EXPERIMENTS. 49 



hand ii'og ; these niiinbers were the mean of nine trials. 

 With the left eye alone the results Avere, right-hand cylin- 

 der 1078, left-hand ii'oi, being the mean of nine trials. 

 Thus, using one eye only, the results are nearly the same in 

 both cases ; they also tend to made the right hand a little 

 less, thus reversing the case of two eyes. These experiments 

 were made in a room with a small window facing the south. 

 I afterwards repeated the experiments, using two ejes, in 

 another room having a window of larger dimensions and 

 facing the north. A solution was used containing 1600 ; 

 the mean of nine trials gave, right-hand cylinder ii*i6, 

 left-hand 10*55, nearly the same results as I got before. 

 Why I should have this tendency to make one column a 

 little longer than the other I do not know ; possibly it may 

 be some peculiarity of vision confined to myself. In the 

 course of my experiments I have also noticed the following 

 curious phenomenon ;, and this repeatedly, when working 

 with solutions coloured with bichromate of potash and with 

 ammonio-sulphate of copper : — Look steadily with one eye 

 (say, the right) through the solution at a white surface, after 

 the lapse of about a minute suddenly turn the head so as 

 bring the left eye close over the cylinder ; then the colour 

 will seem more intense than it did with the right. Having 

 looked with the left eye for about a minute, bring again 

 the right eye suddenly close over the cylinder, and the 

 colour will seem more intense than it did with the left, and 

 so on alternately. It would seem as if the first impressions 

 of colour on the eye were the stronger, and as if there were 

 a gradual and imperceptible decrease in intensity. Per- 

 haps alterations in the aperture of the pupil may contribute 

 to this. 



Another matter for consideration in colorimetry is the 

 nature of the incident light. On some occasions we have 

 the light from a blue sky ; on other occasions the sky is 



SER. III. VOL. VII. E 



