INFRACTION AND DISPERSION OF THE HALOGENS, HALOGEN ACIDS, ETC. 7 



The region of which the refractivity was measured by us extended from \ 6708 to 

 X 4799'9 and thus covers the whole range of the line spectrum and 200 A.U. which 

 are affected by the general absorption. 



It is, therefore, of interest to find that the observed values of the refractivity lie on 

 a smooth curve. But it is significant that the calculated curve cuts across the 

 experimental, in which the curvature is greater. This appears to indicate that a 

 single term formula is inadequate to express the results, and that a second term is 

 required, in which both N and n* are small, to represent the influence of the 

 absorption band. 



In the region of the line spectrum it was to be expected that if any variations of 

 refractive index accompanied the variations of absorption they would be found either 

 in the immediate neighbourhood of each dark line or possibly affecting the whole 

 breadth of each group of lines forming a fluting. In order to investigate this point 

 the following test was made : 



The paths of the two interfering rays of light were equalised, so that when the 

 wiive-length of the light employed was continuously changed from red to violet no 

 rh.mge was observed in the position of the interference bands in the field of view. 

 Chlorine was then admitted into one tube till the path of that beam had been retarded 

 by 450 bands (\ = 5461). Next, by means of the compensator,* the same beam was 

 accelerated by an equal amount. If, now, the wave-length of the light be changed 

 from red to violet any movement of the bands would be due to the difference 

 of refractivity of glass and chlorine for the particular wave-length which is in the 

 field of view. The bands can easily be read to 1/10, so that when 450 bands have 

 passed a difference of refractivity of 1/4500 can be detected. 



The slit was then narrowed till the interference systems due to X 5790 and X 5769 

 were clearly separated in the field of view : i.e., till the light composing any particular 

 part of the image varied by less than 20 A.U. On changing the wave-length 

 continuously from red to violet no sudden change in the bands could be detected. It 

 may therefore be concluded that between X 6708 and X 4799 any sudden change of 

 refractivity exceeding '000784/4500 = '0000017 must te confined to a breadth of less 

 than 20 A.U. and probably to less than half that amount. It is not possible to detect 

 small changes in the refractivity in a narrower section of the spectrum than this, since, 

 if the light is sufficiently dispersed, it becomes too feeble to read tenths of a I >;i i H I. 



BROMINE. 



Previous Work. DUFKT records MASCART'S value n = 1 '001 125 for the D line. 

 The dispersion of the gas has, apparently, not been attempted. 



Preparation. The purest bromine obtainable from Kahlbaum was used. Before 

 every experiment the bulb containing the liquid was cooled to 80 C. and exhausted, 



h This compensator, of special construction, retards all wave-lengths equally except in so far as 

 dispersion affects them. 



