858 Lord Rayleigh on an Improved Apparatus 



wires will be seen on a different part of the spectrum after 

 each half period. In order to fix the new part of the spectrum 

 upon the cross wires, a change of focus is demanded. This 

 experiment would hardly succeed with properly compensated 

 object-glasses, but it could be imitated with the aid of single 

 lenses. 



XLI. On an Improved Apparatus for Christiansen's Experi- 

 ment. By Lord Rayleigh, F.R.S.* 



THE very beautiful experiment in question, described by 

 C. Christiansen in Wiedemann's Annalen for November 

 1884, consists in immersing glass-powder in a mixture of 

 benzole and bisulphide of carbon of such proportions that for 

 one part of the spectrum the indices of the solid and of the 

 fluid are the same. Being interested in this subject from 

 having employed the same principle for a direct-vision spec- 

 troscope (Phil. Mag. January 1880, p. 53), I have repeated 

 Christiansen's experiment in a somewhat improved form, 

 which it may be worth while briefly to describe, as the matter 

 is one of great optical interest. 



I must premise that the beauty of the effect depends upon 

 the correspondence of index being limited to one part of the 

 spectrum. Rays lying within a very narrow range of refran- 

 gibility traverse the mixture freely, but the neighbouring- 

 rays are scattered laterally much as in passing ground glass. 

 Two complementary colours are therefore exhibited, one by 

 direct, and the other by oblique, light. In order to see these 

 to advantage, there should not be much diffused illumination, 

 otherwise the directly transmitted monochromatic light is 

 liable to be greatly diluted. The prettiest colours are ob- 

 tained when the undisturbed rays are from the green ; but 

 the greatest general transparency corresponds to a lower 

 point in the spectrum. 



The improvement referred to relates merely to the use of a 

 flat-sided bottle to contain the preparation. In order to get 

 a satisfactory result it is necessary that the sides of the con- 

 taining vessel be pretty good optically. This condition may 

 be satisfied with a built-up cell, but on account of the diffi- 

 culty of finding a suitable cement, it is rarely that such 

 cells remain in good order for any length of time. It 

 occurred to me that a bottle might be made to answer the 

 purpose, provided the precaution were taken of using the same 

 kind of glass for the bottle and for the powder. The outer sur- 



* Communicated by the Author. 



