82 Lord Rayleigh on Achromatic 



that is necessary is to hold the magnify er somewhat excentri- 

 cally. The bands may then be observed gradually to detach 

 themselves from the mirror, until at last the complete system 

 is seen, as in FresneFs form of the experiment. 



If we wish to observe interference under high relative 

 retardation, we must either limit the light passing the first 

 slit to be approximately homogeneous, or (after Fizeau and 

 Foucault) transmit a narrow width of the band-system itself 

 through a second slit, and subsequently analyse the light into 

 a spectrum. In the latter arrangement, which is usually the 

 more convenient when the original light is white, the bands 

 seen are of a rather artificial kind. If, apart from the hetero- 

 geneity of the light, the original bands are well formed, and 

 if the second slit be narrow enough, the spectrum will be 

 marked out into bands ; the bright places corresponding to 

 the kinds of light for which the original bands would be 

 bright, and the black places to the kinds of light for which 

 the original bands would be black. The condition limiting 

 the width of the second slit is obviously that it be but a 

 moderate fraction of the width of a band (A). 



If it be desired to pass along the entire series of bands up 

 to those of a high order by merely traversing the second slit 

 in a direction perpendicular to that of the light, a very long 

 mirror is necessary. But when the second slit is in the region 

 of the bands of highest order (that is, near the external limit 

 of the field illuminated by both pencils), only the more distant 

 part of the mirror is really operative ; and thus, even though 

 the mirror be small, bands of high order may be observed, if 

 the second slit be carried backwards, keeping it of course all 

 the time in the narrow doubly-illuminated field. In one 

 experiment the distance from the first slit to the (3-inch) 

 reflector was 27 feet, while the second slit was situated behind 

 at a further distance of 4 feet. The distance (6) between the 

 first slit and its image in the reflector (measured at the 

 window) was about 13 inches. 



As regards the spectroscope it was found convenient to use 

 an arrangement with detached parts. The slit and collimating 

 lens were rigidly connected, and stood upon a long and rigid 

 box, which carried also the mirror. The narrowness of the 

 bands in which this slit is placed renders it imperative to 

 avoid the slightest relative unsteadiness or vibration of these 

 parts. The prisms, equivalent to about four of 60°, and the 

 observing telescope were upon another stand at a little distance 

 behind the box which supported the rest of the apparatus. 



Under these conditions it was easy to observe bands in the 

 spectrum whose width (from dark to dark) could be made as 



