

the Colour- Sequence in the Spectrum. 219 



retarding plate with its edge also vertical and covering half 

 the aperture Q. Two alternative positions o£ this plate are 

 shown by heavy and dotted lines at P 1? P 2 . For observing 

 •Talbot's bands the slit S should be removed or opened wide. 



Fier. 4. 



-E=EZEU$ 



It will then be found that, with the plate in position P 1? the 

 spectrum appears channelled by dark and bright bands, 

 while with the plate in the position P 2 the spectrum is 

 uninterrupted. One advantage of the arrangement described 

 here is that Talbot's bands can be observed under the con- 

 dition of best visibility for very small retardations of the 

 order of 4 or 5 wave-lengths without unduly restricting 

 the aperture through which the light has to pass, or reducing 

 the dispersive power of the spectroscope employed. All that 

 is necessary for this purpose is to increase the distance L S, 

 which can be done by bringing the lens L and the plate P 

 nearer to the slit 0, so that the diffraction-pattern at S may be 

 formed on a larger scale. The colour-sequence in Talbot's 

 bands may be very well observed with this arrangement, and 

 is best seen when a diffraction-grating of a few thousand 

 lines per inch is used as the dispersing apparatus, instead of 

 a prism as shown in the figure. If the aperture Q is small, 

 the lens L may be dispensed with, but this is not so satis- 

 factory. 



Another interesting feature of the arrangement described 

 above is that it serves admirably to illustrate the theory of 

 the formation of Talbot's bands. This is shown by studying 

 the effect of narrowing the slit S of the spectroscope so that 

 the whole of the diffraction-pattern formed at the focus of 

 the lens L cannot enter the dispersing apparatus. In this 

 case, if the slit be very narrow, the interference bands in the 

 spectrum are seen equally well in either of the two positions 

 of the plate, the colours seen being those of the pure spec- 

 trum (figs. 2 b and 3 b). On gradually opening the slit S, 



