the Colour- Sequence in the Spectrum. 



215 



uniform colour of the band except perhaps near the edges 

 where the total illumination will be very small. If, how- 

 ever, the dispersion is still less, the maxima for smaller 

 wave-lengths will lie more to the right and produce the 

 reversal of the sequence of colour within a band. 



Fig. 1. 



The colours at different points in a spectrum showing 

 Talbot's bands have been worked out in an actual case with 

 five bands, the retardation of the plate being nine wave- 

 lengths for \ = 5870, the spectrum being normal, and the 

 dispersion adjusted so that the condition for uniform colour 

 stated above is satisfied in the neighbourhood of this wave- 

 length. The assumed law of dispersion gives 



\= 5870 -162-4/17, 



the unit of abscissa being such as to make 



lllZ = X . y 



for \=5870. 



