SOME COLORIMETRICAL EXPERIMENTS. 265 



and endeavouring to arrange these solutions in their order, 

 the arrangement being made according to the judgment of 

 the tints. The various successes attending such exercises 

 would afford the experimenter some method for testing 

 his skill from time to time ; it would also show him under 

 what conditions he could use neither this nor any other 

 method of colorimetry ; for when he has arrived at such a 

 stage that he can perceive no difference of tint, notwith- 

 standing an increase in the colouring-matter, it seems as 

 useless to try to do so as to use a balance of coarse sensi- 

 bility to distinguish between two weights which are very 

 nearly the same. By frequent exercise and concentrated 

 attention, I think, the power of perception o£ difference of 

 shade may be increased. If such a series of cylinders were 

 taken, containing, say, quantities of bichromate of potash, 

 increasing from i to 30, the first members of the series 

 would be easily arranged, towards the middle of the series 

 the number of successful discriminations would diminish, 

 and few would be able to detect differences in the higher 

 members of the series ; for an increment of the salt which 

 produces a considerable difference in intensity when a 

 small quantity of the salt is present produces a small dif- 

 ference when the quantity is large. This would follow 



dt c 



from the approximate formula used; for;7^=— ^ {t=. 



depth, Q = quantity of colouring-matter, c = constant) ; so 

 that, for a given small difference in the quantity, the dif- 

 ference in the depth would be nearly proportional to the 

 inverse square of the quantity. Solutions whose quantities 

 of colouring-matter are so similar as to be beyond discri- 

 mination by colorimetry may be again brought within its 

 range by the following device : in each of the the two 

 cylinders destroy by some appropriate method an equal 

 quantity of the colouring-matter ; then their differences 



