224 Talbot's Bands and Colour-Sequence in the Spectrum. 



minima of illumination. The spectrum with Talbot's bands 

 is thus shown to consist not as usual of an infinite variety 

 of tints, but of a limited number only, depending on the 

 number of bands present. 



3. It is found that the condition for observing this pheno- 

 menon is not the same as that required for observing Talbot's 

 bands at best visibility, i. e. with perfectly black minima, 

 but is slightly different, the dispersion required in the 

 former case being about J of what is needed in the latter. 

 If the dispersion is less than even this the sequence of colour 

 within a band is actually reversed. 



4. The colour at every point in a spectrum showing five 

 Talbot's bands has been calculated and the result shown on a 

 graph and plotted on Maxwell's Colour-Triangle. 



5. A new method for observing Talbot's bands is described, 

 which consists briefly in first forming the laminar diffraction- 

 pattern in white light and then analysing it with a spectro- 

 scope having a wide slit. This method is very convenient 

 when Talbot's bands with small retardations have to be 

 observed, as it is then unnecessary to unduly restrict the 

 aperture or to use very small dispersive powers. 



6. With the foregoing arrangement it is possible also to 

 study the manner in which the visibility of Talbot's bands 

 is influenced by admitting more or less of the laminar 

 diffraction-pattern into the dispersing apparatus, and thus to 

 actually trace the successive stages of the process of super- 

 position which results in the interference bands having full 

 visibility when the plate is put on the thinner side of the 

 dispersing prism, and zero visibility when it is on the 

 thicker side. 



7. It would be interesting to see if the phenomenon of the 

 colour-sequence in Talbot's bands could be explained on the 

 simple theorv of their formation which has been given by 

 Schuster (Phil. Mag. Jan. 1904). 



In conclusion, the writer has much pleasure in recording 

 his sense oE deep gratitude to Prof. C. Y. Raman for his 

 valuable and helpful interest during the progress of the 

 work which was carried out in the laboratory of the Indian 

 Association for the Cultivation of Science. 



Calcutta, 8th July, 1920. 



