THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



AUGUST 1889. 



XL On Achromatic Interference- Bands. By Lord RAYLEIGH, 

 Sec. U.S., Professor of Natural Philosophy in the Royal 

 Institution*. 



Introduction . 



WHEN there is interference of light, the width of the 

 resulting bands, measured for example from darkness 

 to darkness, is usually a function of the colour of the light 

 employed. Thus, in the case of Fresnel's well-known inter- 

 ference-experiment, in which light reflected from two slightly 

 inclined mirrors illuminates a screen, the width of the bands 

 is proportional to the wave-length of the light. In order that 

 a considerable number of bands may be visible, it is necessary 

 that the light be highly homogeneous ; otherwise it is impos- 

 sible that the various band-systems can fit one another over the 

 necessary range. If the light could be supposed to be abso- 

 lutely homogeneous, there would be no limit to the number 

 of observable bands : and, what is especially to be remarked, 

 there would be nothing by which one band could be distin- 

 guished from another, — in particular there could be no central 

 band recognizable. When, on the other hand, the light is 

 white, there may be a central band at which all the maxima 

 of brightness coincide ; and this band, being white, may be 

 called the achromatic band. But the system of bands is not 

 usually achromatic. Thus, in Fresnel's experiment the centre 

 of symmetry fixes the position of the central achromatic band, 



* Communicated "by the Author. 

 Phil. Mag. S. 5. Vol. 28. No. 171. August 1889. H 



