DR. JAMES BOTTOMLEY ON COLORIMETRY. 67 



little inwards. Measurements were taken from the summit 

 of the curve. When in such cylinders we have short 

 columns of fluid, the depth not being uniform, the colour 

 is not uniform over the whole area as we look through the 

 cylinder at an external white surface. Manifestly the 

 colour at the sides is more intense than at the middle ; but 

 for purposes of comparison we must restrict our attention 

 to the middle. It is not easy to confine attention to a 

 limited portion of a coloured area, so as to receive no im- 

 pression from the remainder of the area, without some 

 provision. Hence it is necessary to limit the field of view 

 at the bottom of the cylinder. This was done either by 

 placing small porcelain disks on a black ground and holding 

 the cylinder so that its axis passed through the centre of 

 the disk, or, still better, by covering the bottom of the 

 cylinder with a black external plate having a small hole 

 (about a quarter of an inch diameter) in its centre. With 

 such a provision, columns seemed in some cases to satisfy 

 the experiment which otherwise would have given the im- 

 pression of too dark a colour. In these experiments I 

 used a method for determining colours indicated in my last 

 paper, regarding the proper colour as the mean of two sets 

 of determinations, one set giving too great and the other 

 too small values. Thus the determination of colour has 

 some analogy with the method used by old geometers for 

 determining areas bounded by curved lines ; considering 

 them as the limits of internal and external polygons. In 

 these experiments A denotes the number of cubic centi- 

 metres of caramel solution mixed with water, B the length 

 of the column, and C the number of cubic centimetres 

 thence derived by calculation. In one experiment the mean 

 of four trials for the greater limit gave 2*83 cm. ; and the 

 mean of four trials for the smaller limit gave 2*65. Hence 

 the result will be as follows (standard solution contains 



p2 



