86 Lord Rayleigh on Achromatic 



observed in the spectrum of light transmitted through w 2 , 

 whose brightness is proportional to n~ 1 , instead of, as before, 

 to n~ 2 . 



Achromatic Interference- Bands. 



We have already seen from (3) that in the ordinary arrange- 

 ment, where the source is of white light entering through a 

 narrow slit, the heterogeneity of the light forbids the visibility 

 of more than a few bands. The scale of the various band- 

 systems is proportional to X. But this condition of things, 

 as we recognize from (2j, depends upon the constancy of b, 

 that is, upon the supposition that the various kinds of light all 

 come from the same place. Now there is no reason why such 

 a limitation should be imposed. If we regard b as -variable, 

 we recognize that we have only to take b proportional to X, 

 in order to render the band-interval (A) independent of the 

 colour. In such a case the system of bands is achromatic, and 

 the heterogeneity of the light is no obstacle to the formation 

 of visible bands of high order. 



These requirements are very easily met by the use of 

 Lloyd's mirror, and of a diffraction-grating, with which to 

 form a spectrum. White light enters the dark room through 

 a slit in the window-shutter, and falls in succession upon a 

 grating, and upon an achromatic lens, so as to form a real 

 diffraction- spectrum, or rather series of such, in the focal 

 plane. The central image, and all the lateral coloured images 

 except one, are intercepted by a screen. The spectrum 

 which is allowed to pass is the proximate source of light in 

 the interference experiment ; and since the deviation of any 

 colour from the central white image is proportional to X, it is 

 only necessary so to arrange the mirror that its plane passes 

 through the white image in order to realize the conditions for 

 the formation of achromatic bands. 



There is no difficulty in carrying out the experiment practi- 

 cally. I have used the spectrum of the second order, as given 

 by a photographed grating of 6000 lines in an inch, and a 

 photographic portrait lens of about 6 inches focus. At a 

 distance of about 7 feet from the spectrum the light fell 

 upon a vertical slab of thick plate-glass 3 feet in length and 

 a few inches high. The observer upon the further side of the 

 slab examines the bands through a Coddington lens of some- 

 what high power, as they are formed upon the plane passing 

 through the end of the slab. It is interesting to watch the 

 appearance of the bands as dependent upon the degree in 

 which the condition of achromatism is fulfilled. A com- 

 paratively rough adjustment of the slab in azimuth is sufficient 



