COLORIMETRICAL TiXPERIMENTS. 47 



lution containing 2400^ the length of column was 12 ; 

 from the uncorrected formula the result is 2824 ; from 

 the formula 



gr'(i2 + 4-8) =i6oo(2i-2 + 4-8) 



the resulting value of q is 2476_, which is not far from the 

 proper value. 



When the fluids compared diflfer much in strength^ the 

 value of the collection will probably vary. It will be 

 better not to have the difference large, whether the disks 

 be external or internal ; for when the differences are large, 

 any errors in the determination of the lengths of the 

 columns have a greater effect on the calculated result. 



I also tried to make a rough estimate of the length of 

 the cylinder which might be covered without any percep- 

 tible darkening of the disk. I found that a black cloth 

 cover investing the cylinder, might be drawn down until it 

 was about 3*5 or 3*2 centims. from the disk ; this would 

 vary with the dimensions of the window and the relations 

 of the cylinder to it. Also the length given is less than the 

 value of X previously deduced : but it ought to be so ; for 

 the light from a vertical window to illuminate a horizontal 

 disk must pass obliquely through the solution. It would 

 also follow that parts o£ the disk more remote from the 

 window are darker than parts nearer ; hence, if the cylin- 

 ders are of moderate radius, either the disks should be small 

 and should be kept with their centres moving along the 

 axes of the cylinders, or, in the case of larger disks, the de- 

 termination of colour should be made by a comparison of 

 similar parts of the disks. 



In the estimation of colour, it is also not a matter of 

 indifference, when we are given any tint as a limit, how 

 we approach that limit. Suppose we have two cylinders, a 

 and ^, full of coloured liquid, that in a being the darker. 



