OF LIGHT BY TURBID SOLUTIONS. 197 



resulting tints equal. The lengths of the columns were 

 made ii and i6'2; I thought they were about equal, pos- 

 sibly "VYa slightly lighter. My impression varied a little 

 with the illumination of the surfaces. The lengths of the 

 columns were now made 15 and 20*2; the tints seemed 

 about equal, possibly Wa slightly lighter. The difference, 

 if any, must be small, as is shown by the following experi- 

 ment. Taking Wb seen through a column 15 cm. long 

 as the standard tint, I endeavoured to get the same tint 

 with Wa seen through the other cylinder. For the lower 

 limit I got i9'9 cm., and for the upper limit 21 cm.; the 

 mean of these is 20"45, not far removed from 20* 2. After- 

 Avards I thought a column of this length satisfied. I now 

 made the lengths of the colmmns 19 and 24*2 ; I thought 

 that the tints again corresponded. 



Some experiments of a similar nature were made with a 

 stronger diffusion, it contained 3747 cub. c. in 500 cub. c. 

 The tints of grey used were the same as in the last experi- 

 ments. The standard tint at the commencement was Wb 

 seen through a column 4 cm. long. To get a similar tint 

 with Wa one determination for the upper limit gave 6-5, 

 and one determination for the lower limit gave 6*i. The 

 mean of these is 6*3. A column of this length seemed to 

 satisfy. The common increment is 3 cm. The columns 

 were made 7 and 9*3 cm. long ; the tints seemed to 

 correspond. The columns were made 10 and 12*3 cm. 

 long ; the tints again seemed to correspond. The columns 

 were made 13 and 15 "3 cm. long; the tints seemed 

 again to correspond. Finally the columns were made 16 

 and 18-3 cm. long. The tints again corresponded. In 

 the last two experiments the greys obtained were very 

 deep. I also made the following experiment : — I took 

 two grey tints : one consisted of 10 grms. BaSO^ and 

 0'042 grm. of lamp-black; the other consisted of 10 



