200 Lord Rayleigh on Copyiny Diffraction-gratings, 



and became alternately blue and yellow. When the lens was a 

 little more removed, the bands again became red and green. 

 And this change continued to take place for an indefinite 

 number of times, as the distance between the lens and grating- 

 increased. In all cases the bands exhibited two comple- 

 mentary colours. 



rt It was very curious to observe that, though the grating- 

 was greatly out of the focus of the lens, yet the appearance of 

 the bands was perfectly distinct and well defined. 



" This, however, only happens when the radiant point has 

 a very small apparent diameter, in which case the distance of 

 the lens may be increased, even up to one or two feet from the 

 grating, without much impairing the beauty and distinctness 

 of the coloured bands. So that if the source of light were a 

 mere mathematical point, it appears possible that this distance 

 might be increased without limit ; or that the disturbance in 

 the luminiferous undulations caused by the interposition of 

 the grating continues indefinitely, and has no tendency to 

 subside of itself/'' 



It is scarcely necessary to point out that what was seen by 

 the eye in this experiment in any position of the magnifying 

 lens was the same as would have been depicted upon a photo- 

 graphic plate situated at its focus, at least if the same kind of 

 rays had been operative in both cases. Talbot's observa- 

 tions are therefore to the point as determining the effect of 

 varying intervals in photographic copying. 



On the whole the above description agrees well with what 

 I had expected from theory. It is indeed impossible to admit 

 that the red and green coloration could disappear and revive 

 an indefinite number of times. The appearance of colour at 

 all shows that the phenomenon varies with the wave-length, 

 and accordingly that it would (as in all such cases when white 

 light is used) ultimately be lost. Besides the limit imposed 

 by the apparent magnitude of the source of light, there must 

 be another depending upon the variation of wave-length 

 within the range concerned. 



In trying to repeat Talbot's experiment I found that even 

 the 3000-to-the-inch grating was too fine to be conveniently 

 employed ; and eventually I fell back upon a very coarse grating 

 made some years ago by photographing (with the camera and 

 lens) a piece of striped stuff. By comparison of coincidences 

 with the divisions of fine ivory scale (vernier fashion), the 

 period was determined as *0104 inch. As a source of light 

 I used a slit placed parallel to the lines of the grating, and 

 backed by a fish-tail gas-flame seen edgeways. In order to 

 observe the appearances behind the grating, a lens of moderate 



