Z DR. JAMES BOTTOMLEY ON THE 



radius, may be remedied by covering the bottom with a 

 black plate having a small aperture in the centre ; in this 

 way the rays coming from the sides are cut off. But, 

 even supposing this done,, the colour will not be exactly 

 the same over the whole area. Suppose an eye to be 

 placed at A in the axis of the cylinder, and let C H repre- 



4 



I 



A 



1 





B 



CGDE H 



sent the section by a vertical plane of the circular area at 

 the bottom admitting light. Let A E be the ray which 

 passes through the centre ; the path of this ray through 

 the liquid will be EB. Let AEG be another ray which 

 reaches the eye ; the path of this ray through the 

 liquid is F G ; and as this is greater than E B, the ab- 

 sorption will be greater. Hence the colour will gradually 

 increase in intensity as we pass from the centre to the 

 circumference. For colorimetric measurements we take 

 the vertical length of the column of liquid. In what 

 follows I propose to consider if this would lead to any 

 noticeable error. Our impression of colour will not be 



