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XVIII. On Talbot's Bands and the Colour- Sequence in the 

 Spectrum. By Nihal Karan Sethi, M.Sc, Assistant 

 Professor of Physics in the Benares Hindu University *. 



1. Introduction. 



THE theory of the very remarkable system of bands in 

 the spectrum discovered by Talbot which is seen on 

 covering half the aperture of the dispersing system by a 

 retarding plate has been discussed by a number of writers, 

 notably by Airy f, Stokes J, Rayleigh §, Schuster ||, Walker H, 

 and Wood **. The treatments given make it clear that a 

 spectrum in which the bands are seen is less pure than it 

 would be in the absence of the retarding plate — in other 

 words, that its introduction results in a re-distribution of 

 energy in the different parts of the spectrum as actually 

 formed. But the effect of such re-distribution of energy on 

 the sequence of colours as seen by the eye in a spectrum 

 showing Talbot's bands, does not so far appear to have been 

 considered f j\ Some observations made by the present 

 writer show that the colour-sequence in Talbot's bands pre- 

 sents some very remarkable features, which become parti- 

 cularly striking and noticeable when the total number of 

 bands in the visible spectrum is not large. It is proposed 

 in this paper to describe the methods of observation used and 

 the results obtained, and incidentally also to discuss certain 

 other aspects of the theory of formation of Talbot's bands. 



2. Observation of the Colours of Talbot' s Bands. 



The human eye is in reality a very sensitive instrument for 

 detecting changes in colour, and, as was pointed out by the 

 late Lord Rayleigh f if, it is capable under favourable con- 

 ditions of detecting as small a difference of tint as that which 

 exists between the two D-lines of sodium. The eye fails,. 



* Communicated by Prof. C. V. Raman, M.A. 



t Phil. Trans, i. p. 1 (1841). 



X Phil. Trans, ii. p. 227 (1848) ; also Math. & Phys. Papers, vol. ii. 



§ Scientific Papers, vol. lii. p. 123. 

 || Phil. Mag. January 1904, p. 1. 

 U Phil. Mag. April 1906, p. 531. 

 ** Phil. Mao-. November 1909, p. 758. 



tt -Ft> r a full bibliography of the literature on Talbot's bands, see 

 Appendix to a paper by T. E. Doubt, Phys. Rev. Oct. 17, p. 332. 

 XX Scientific Papers, vol. v. p. 621. 



P 2 



