the Refractive Index and Dispersion of Glass. 415 



other useful immersion fluids. In its most concentrated 

 form the refractive index of: the solution for the D line is 

 1*72, and the index may be varied continuously down to 1"33 

 by admixture with increasing proportions of water. The 

 solution possesses the great advantage from the practical 

 point of view in that the excels of water may be driven off 

 by heat and the original solution with a high index of 

 refraction recovered again. Many of the mixtures that 

 have been suggested for the purpose of obtaining a solution 

 of variable refractive index involve the employment of two 

 liquids other than water which cannot be separated and 

 recovered in this way after use. The double iodide possesses 

 the further useful property that it does not attack the 

 Canada balsam used for cementing together the walls of the 

 cell. The main drawback to the use of the solution for the 

 present purpose lies in the fact that it exercises marked 

 absorption at the more refrangible end of the spectrum. 



Fig. 3. 



BC 



2: 















^- 











/ 

















\y 











4. 



JOfJfj 5 



OO^ju 5 



SOp/j 6 



OOf/fj 6. 



JOfjfj 



Wave length 



The transmission curve for light of various wave-lengths 

 passing through the cell filled with Thoulet solution of 

 refractive index ra D =l'51 was obtained by means of a spectro- 

 photometer, with the result shown in fig. 3. 



It will be seen that the percentage transmissions for the 

 four customary lines C, D, F, and Gr' are approximately 70, 



