124 



Dr. T. Ewan. On the 



justment was generally defective, and was repeated. As the apparatus 

 was not very rigid, owing to its size, the adjustments had to be 

 repeated every two or three weeks. That of the tube of solution was 

 done before each set of measurements, and the wave-lengths of the 

 light were read after each day's work. The following readings were 

 made on a solution of CuS0 4 in the tube 1 meter long ; they are read- 

 ings of average goodness, and will serve to show the sort of differences 

 which occurred between the single readings. The numbers given are 

 the openings of the sector. 



Reduction of Results. 



The quantity which is directly measured is the amount of light 

 which is lost by passing through a system consisting of two glass 

 plates with a layer of aqueous solution between them. In order to 

 calculate from these numbers the part of the light absorbed by the 

 dissolved substance, it is necessary to allow for the light reflected or 

 absorbed by the glass plates and the water. To obtain the necessary 

 formulae, I have applied the equations given by Stokes ('Boy. Soc, 

 Proc.,' vol. 11, p. 545) to this special case as follows : 



Call the fraction of the light incident on the glass plate from air, 



which is reflected p = 



- r 



+ 1 



, and the same quantity when the 



light is incident from the liquid on the glass plate/)' = f^, ~~ V 



/i and fi are the refractive indices for air to glass and water to glass 

 respectively. And suppose that a fraction (1 a) of the light is 

 absorbed by passing once through the glass plate. Then, allowing 

 for the repeated reflections between the two surfaces of the plate, the 

 quantity of light (calling its original intensity = 1) which will pass 

 into the liquid is 



t = 



