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



however, to perform this important function in an adequate 

 manner when a large number oti different colours are pre- 

 sented to it simultaneously without sharp lines of separation, 

 as, for instance, in a prismatic spectrum. But if the colours 

 are presented to the eye separately, and occupying a fairly 

 large area in the field of view, its power in this respect 

 becomes surprisingly great. These facts have to be borne in 

 mind in attempting to discriminate between the sequence of 

 colours in an ordinary spectrum and the sequence as seen in 

 a spectrum showing a considerable number of Talbot's bands. 

 The following method of observation has been found to be 

 suitable. The eyepiece of the observing telescope is removed 

 and a narrow slit is placed in the focal plane of the objective. 

 On putting the eye immediately behind the slit, the whole of 

 the effective portion of the prism face is seen to be of one 

 uniform colour, and the field of view is also considerably 

 broadened by the diffraction of the light entering the eye 

 through the slit. On moving the telescope by slowly turn- 

 ing the tangent screw, the colour of the different bands 

 and even of the different portions of a band can be easily 

 examined. 



Having chosen, then, a mica plate of such thickness that 

 it gave about 25 or 30 bands in the whole spectrum, the 

 aperture of the beam was carefully adjusted till, with the 

 help of a nicol, the visibility of the bands became about 

 the best. The eyepiece was now replaced by the slit men- 

 tioned above, and it was observed on turning the tangent 

 screw of the telescope that the colour seen changed in a 

 remarkable manner. It remained almost unaltered except 

 in intensity throughout the width of a bright band, but in 

 crossing a minimum, there was practically a sudden jump 

 to another colour quite distinct from the first. This again 

 persisted unchanged in tint until the next minimum was 

 reached, when there was another sudden change, and so on. 

 The gradation of colour and the almost imperceptible passage 

 from one tint to the next observed in an ordinary spectrum 

 was entirely lost. The spectrum showing Talbot's bands was 

 thus seen to consist not of an infinite variety of tints as 

 found in the ordinary spectrum, but of a limited number 

 only. It is needless to add that this remark applies strictly 

 to that part of the spectrum only for which the adjustment 

 for this purpose is perfect. 



