1 43 
5*4 is not very far from it. When I actually tried a column 
of the theoretical length it seemed to give the required tint 
when the standard solution was on the left; when the 
standard solution was on the right it seemed a little darker. 
The mean of the two values previously obtained is 5968, 
which is near to the real value. From the above experi- 
ments it seems that when excesses of ammonia are added, 
very fair approximations may be obtained by colorimetry 
to the quantity of copper sulphate in solution, the white 
surfaces being external. Also that with white surfaces 
internal the results are more remote. In my last paper I 
stated that when there was much difference between the 
standard solution and the one to be compared with it, the 
discrepancies when internal discs were used were consi- 
derable. Then I was using very small quantities of colour- 
ing matter. From these experiments where the colours 
were intense and the quantities of colouring matter used 
considerable, a similar conclusion follows. The reason for 
this is not far to seek, and it also suggests a correction that 
must be applied to the formula when the white surfaces are 
inside. A white disc inside a column of coloured liquid 
looks darker in colour than a white disc outside, placed a 
few inches below a column of the same length. This may 
be tried by looking through different columns, or through 
the same column, and having inside a white disc of smaller 
diameter than the cylinder ; the inside disc will then appear 
surrounded by a rim of lighter colour. When the disc is 
internal, it is evident that the light which illuminates it has 
previously passed through the solution, so that we are look- 
ing not at a white disc, but at a coloured disc through a 
coloured solution. Some allowance must be made in the 
calculation for this coloration of the disc. The formula 
qt = qt' is applicable to the case in which the surfaces are 
outside ; to adapt this formula to the case when the discs 
are inside, suppose x to be the length of the column of fluid 
