416 Mr. R. W. Cheshire on a New Method of Measuring 



63, 16, and 2 respectively. The transmission curve shown 

 in the figure relates to the Thoulet solution actually employed 

 in the cell for these refractive index determinations. Owing 

 to a tendency of the iodine to separate out from the solution 

 the tint of a solution of given index is apt to vary and to 

 pass from a yellowish-green to a brown colour as this 

 separation increases. The corresponding transmission curves 

 will therefore change slightly, but will retain the same 

 general characteristics as regards the spectrum distribution 

 of intensity in the transmitted light. The original yellow- 

 green solution may always be recovered again by warming 

 the brown solution in the presence of a little clean mercurv. 

 In common with all liquids the temperature coefficient 

 of the refractive index of Thoulet's solution is very high, 

 and is of the order of —0*0006 per degree centigrade. It 

 might have been expected that this circumstance would, in 

 itself, preclude the possibility of obtaining any significant 

 result to five figures, but in practice no difficulty is 

 experienced if the apparatus be shielded frotn direct currents 

 of air. After a balance has been obtained through the 

 observing telescope for the equality of the indices, an 

 interval of not more than a minute need elapse before two 

 or three readings have been taken for the refractive index 

 of the solution, and throughout this interval the solution 

 will not be found to have changed in refractive index to an 

 extent which is appreciable on the Pulfrich refractometer, 

 provided that the solution has previously been allowed to 

 attain the room temperature. In any case two independent 

 observers could, if desired, take simultaneous readings, one 

 on the refractometer, whilst the other views the liquid in 

 the cell through the observing telescope. A much more 

 serious difficulty to be met arises from the unequal evapor- 

 ation at the surface of the liquid and from the walls of the 

 cell, producing local changes in concentration and giving 

 rise to the presence of striae, which eventually permeate the 

 whole mass of the fluid and assume most fantastic forms 

 when seen through the observing telescope with the rest of 

 the field dark. Topler in his original paper describes 

 these striae and gives an excellent reproduction of their 

 appearance. The growth of these beautiful but very un- 

 desirable striae within the fluid may be prevented by pro- 

 viding the open rectangular cell with a glass cover-plate on 

 the under side of which has been smeared a little vaseline. 

 This has the effect of excluding the air, and after a few 

 moments equilibrium is restored between the vapours in the 

 cell and no further local changes of concentration take place. 



