262 DR. JAMES BOTTOMLEY ON 



Another series of experiments was made with the bi- 

 chromate. Standard solution 50 in 500 cub. c. of water 

 (length of column 8-3) : — 



In some of these determinations I felt considerable un- 

 certainty 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 of repeating the experiment on 

 some future occasion. On repetition, I made the depth 

 of the disk 5'8, which would indicate the amount of salt 

 used as 7 1 . In some other instances I felt uncertainty when 

 working with a solution of the bichromate in determining 

 equality of tint ; so that, whether the disk 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. 

 Moreover, when there is a considerable difference between 

 the quantities of the colouring-salt in the two cylinders, 



