the Colour-Sequence in tJie Spectrum. 



22Z 



admits about ^ of the central bright fringe in the diffraction- 

 pattern. Then it reappears with the positions of the dark 



Fig. 5. 



and bright bands interchanged and increases till the slit 

 admits about | of the central portion. The maximum of the 

 visibility in this reappearance is about 16 per cent, of the 

 full visibility. The visibility again vanishes when about J 

 of the central fringe of the diffraction -pattern enters the 

 analysing spectroscope. With further increase of slit width 

 we have another reappearance followed by a disappearance, 

 but these changes can only be seen with difficulty. Actual 

 measurement of the slit width at the different stages showed 

 fair agreement with the results of theory. 



7. Summary and Conclusion. 



1. The present paper contains some observations and a 

 theoretical discussion of the remarkable difference in the 

 colour-sequence as observed in a normal spectrum and in a 

 spectrum showing Talbot's bands. In the case of a spectrum 

 showing a fairly large number of the bands, these differences 

 may be studied by placing the eye behind a very narrow slit 

 placed in the focal plane of the telescope and moving it 

 over the spectrum. With a spectrum showing only a small 

 number of Talbot's bands in the whole visible region the 

 phenomena may be easily seen without such aid. Talbot's 

 bands may be satisfactorily observed with such small retar- 

 dations by using a prism combination of small adjustable 

 dispersion. 



2. It was found in all cases that when adjustments were 

 properly made, the colour changed from band to band in a 

 fairly abrupt manner — remaining nearly uniform within the 

 bright regions, but changing suddenly in passing across the 



