218 Prof. Nihal Karan Sethi on Talbot's Bands and 



owing to the dispersion being less than that required. For 

 the same reason, the minima in this part of the spectrum are 

 not so sharp. 



The above results are much more strikingly evident in 

 figs. 3 a and 3 b, which exhibit the resultant colours in the 

 two cases as points on Maxwell's Colour-Triangle. It will 



Fur. 3. 



lZZkS 



&oiz 



be observed that the points in fig. 3 a cluster together in 

 groups considerably separated from each other, while in 

 fig. 3 6 they are more evenly 

 case of a normal spectrum. 



distributed, exactly as in the 



5. A New Metliod of observing Talbot's Bands. 



In the usual method of forming Talbot's bands by cover- 

 ing half the aperture of the dispersing system by a retar ding- 

 plate, the object-glass of the observing telescope serves the 

 dual function of forming the diffraction-pattern of the light 

 passing through the restricted aperture and of focussing the 

 spectrum formed by the dispersing system. Though this 

 has the advantage of simplicity, it is for certain purposes 

 more convenient and instructive to observe Talbot's bands 

 with an arrangement in which the two processes are sepa- 

 rated, i. e. the diffraction-pattern is first formed in white 

 light and then analysed into a spectrum. 



Fig. 4 represents the disposition of the apparatus required 

 for observing Talbot's bands in the manner indicated above. 

 is a narrow vertical slit whose image is focussed by the 

 lens L on the vertical slit S of the spectroscope. P is the 



