223 
Another series of experiments was made with the bichro- 
mate. Standard solution 50 in 500 cc., length of column 8 ’3. 
A 
B 
C 
60 
6-7 
62 
70 
5-3 
78 
80 
4-5 
92 
90 
4 
104 
100 
3-7 
112 
110 
3-4 
122 
120 
3-2 
130 
130 
3 
138 
140 
2-8 
148 
150 
..... 2-6 
160 
In some of these determinations I felt considerable 
uncertainty at what depth equality of colour was obtained, 
and some of the results were likely to vary on repetition, 
In the table I put 5 3 as the depth after some hesitation, 
and with the intention to repeat the experiment on some 
future occasion. On repetition I made the depth of the 
disc 5 ’8, which would indicate the amount of the salt used 
as 71. In some other instances I felt uncertainty when 
working with a solution of the bichromate in determining 
equality of tint, so that whether the disc were elevated 
above or depressed below the position finally chosen the 
difference in colour was insensible. Perhaps with bright 
yellow colours the eye requires more experience to detect 
differences of shade than with colours not so glaring. More- 
over, when there is a considerable difference between the 
quantities of the colouring salt in the two cylinders there is 
another difficulty to deal with. The colours to be com- 
pared differ not only in intensity but also in kind. A very 
dilute solution of bichromate gives a pale yellow with a 
slightly greenish shade, which passes, as the amount of salt 
is increased, into full yellow, orange yellow, and finally orange. 
A similar observation holds with regard to several other 
colouring salts. Thus a very dilute solution of perman- 
ganate of potash is pink, passing, as the strength of the 
