84 [ftheScr I; M - 3,0 



I M 

 Molecular refraction, - = m . 



For ray C (Ha). 



//- + -2 -/ 



For ray Hy near O. 



a 7 a 7 



KZn sel. 62'7G 63 :?7 64'93 64-33 64'98 6ti 



3-37 3-40 2-99 3*47 3-49 3*05 



Kl.Znsel. 66-13 66'77 67-92 67-80 68-47 69-70 



5-60 5-37 4-77 578 .V.ll 4-88 



CsZnsel. 71 -7:'. 72-14 72-69 73'58 73-98 74-58 



Specific Dispersion, n (i - n c . 



a & 7 



KZn sel 0-0029 0-0030 0-0032 



RbZnsel 0-0027 0-0027 0-0028 



CsZn sel 0-0025 0-0026 0-0026 



Molecular Dispersion, m^ - m t - 



a /3 7 



KZn sel 1-57 % 1-61 172 



RbZnsel 1-67 170 178 



CsZnsel 1-85 1-84 1-89 



// - 1 

 Molecular Refraction (Gladstone), M. 



Q 

 a 7 



KZn sel 107-00 108-24 111-41 



5-90 5-96 5-12 



RbZnsel 112-90 114-20 116-53 



10-16 9-69 8-50 



CsZnsel 123'06 123-89 125-03 



The rules stated in the Abstract regarding specific and molecular 

 refraction are independent of the wave-length, and whether they are 

 calculated by the formulas of Lorenz or Gladstone and Dale. They 

 are also independent of temperature, for it has been shown with 

 regard to each salt that the refraction is diminished by rise of tem- 

 perature, and the density, the other factor in the calculation, is 

 naturally affected in the same direction by increase of temperature. 



The replacement of sulphur by selenium in these zinc double salts is 

 accompanied by an increase of molecular refraction of 7 - 7-4 Lorenz 

 units or 13'0 13'9 Gladstone units for the ray C, according to the 

 direction compared. The increase due to each atom is thus 3-5 3-7 or 

 6-5 6 -9 units. The values derived from a comparison of the simple 

 sulphates and selenates were 3*4 3*8 or 6*2 7 -2. The mean values 

 derived from the two series are thus identical. 



In the next communication the magnesium group of salts Anil be 

 described. 



