10 



MR CLIVE CITHRERTSON AND MAUDE CUTHBERTSON ON THE 



at X = 5600 it was not possible to read more than three, and on the violet side of the 

 region of absorption no measurements were possible. In attempting to measure the 

 dispersion tin- . -\IH-I im.-nt.T has to choose between a small number of bands read over 

 a sli^'htlv wiili-r range and a larger number read over a small range. In either case 

 the errors of observation are relatively large. 



Procedure. A weighed quantity of iodine was introduced into the refractometer 

 tube which was evacuated and sealed off.* 



The tube was then heated till the solid had all sublimed and the bands observed. 



Refraction. The wave-length selected for the absolute determination was 6438, 

 and this was obtained from white light of a Nernst lamp by means of a fixed deviation 

 spectroscope. The volume of the refractometer tube was 49' 1 c.c. and the weight of 

 iodine which it contained was '00473. 



The beet experiments gave for /u-1 the value '00210, and this is probably correct 

 to 1 or 2 per cent. It agrees well with HURION'S value, which was probably for a 

 longer wave-length than 6438. 



Dwpemon. Assuming this value the following numbers were obtained for the 

 refractivity in the red-orange, the number of bands read being 97 for X = 6438. 



TABLE IV. Dispersion of gaseous Iodine. 



In another set of readings the number of bands read was only 2'1 in the red, and 

 the following readings were taken : 



Xx, 0- 6708, G438, 6215, 6,80, 5COO. 5250, 5,00, 5005, 5000 



0.-0.0- . ,970, 2100, 2,30, 2, 30 , 217 0, 2250, 2210, * 1M , 2120 . 



r r at i ~ t th 



, , ug an absorption 



" ch seems to 



, 



... A , vol . 204 , 



