264 DR. JAMES BOTTOMLEY ON 



the same,, and then reading off the result. A solution of 

 bichromate of potash, containing 15 iu 500 cub. c, and 

 depth 8*3, was used as a standard. Solutions containing 

 bo, 70, 80, 90, gave similar tints at the calculated depths. 

 Solutions containing 150 and increasing quantities, gave 

 tints a little too deep at the calculated depths. Also a 

 standard solution of permanganate of potash, containing 

 4*8 in 500 cub. c. of water and depth 8*3, was taken. So- 

 lutions containing 6 and 7*2 gave a similar tint at the depths 

 assigned by the calculation. Solutions containing 9, 9*6, 

 and higher quantities gave tints a little too deep at the cal- 

 culated depths. 



Some of the foregoing results are fair approximations ; 

 and probably better results might be obtained with a more 

 perfect form of apparatus than I employed. The surfaces 

 used were not perfectly flat. Moreover there was in some 

 of the experiments the indecision arising from the coexist- 

 ence of several distinct tints on the surface of the disk, due 

 to the formation of caustic curves. This I afterwards re- 

 medied by working behind a screen of lawn. The effect 

 of this is to render the illumination of the disk much more 

 Tiniform. Instead of a screen we might also use glass cy- 

 linders ground internally. Another difficulty is the inde- 

 cision respecting the depth at which equality of colour 

 takes place. The degree of indecision will be different 

 for different persons, and, I suppose, represents want 

 of sensibility and want of skill in detecting shades of 

 colour. So far as it arises from want of skill, it is likely 

 to be diminished by more perfect experience with colours. 

 It seems to me that it would be well to have some definite 

 method for exercising the eye or for testing one's skill in 

 this respect. This might be done by having a series of 

 cylinders filled with coloured solutions, the quantity of 

 colouring-matter increasing in arithmetical progression, 



