186 DR. JAMES BOTTOMLEY ON COLORIMETRY. 



as miglit be expected, considering the difficulties of tlie 

 inquiry ; and even if all external circumstances necessary 

 for the successful completion of such experiments had 

 existed, there would yet remain the difficulty of deciding 

 about the equality of two grey tints. In such matters it 

 is difficult to say where judgment ends and fancy begins. 



That any discrepancies might be due to such a cause 

 was shown by the following experiments : — I took the two 

 cylinders and poured from one into the other with the in- 

 tention of obtaining the same tint in both. In one trial I 

 could not very clearly distinguish a column i6"4 centim. 

 long from one I4"6 centim. long; and in a second trial a 

 column 1 6*2 centim. long seemed to give the same tint as 

 a column 14" 6 centim. long. In the above experiments 

 I used one eye only, namely the right one. 



I also made the following experiments : — I took as the 

 standard of intensity W seen through a column I2'5 cen- 

 tim. long. On a former occasion I had made 6*2 as the 

 equivalent column to be used with Wj.. On the present 

 occasion I thought 6' 5 centim. gave a nearer result ; so I 

 took the column at this length. Now, if the law of absorp- 

 tion of light be true, if we increase both, columns by the 

 same quantity, the intensities should again correspond. 

 So I added 4 centim. to each, making one column i6'5 

 and the other I0'2. I thought that the tints were the 

 same. I now made one column 20*5 and the other 14" 2. 

 Again I thought the tints were the same. Finally I made 

 one column 24*5 and the other i8'2. The tints seemed 

 again to correspond. 



