the Colour- Sequence in the Spectrum. 213 



3. Observation of Talbot's Bands ivith small 

 Retardations. 



In spite of the convincing nature of the experiment 

 described above, it was felt that the effect would certainly 

 be much more striking if it could be observed without any 

 special aid to the eye and so as to show the different colours 

 side by side at one glance. This would be possible only if 

 the changes were much more sadden, which evidently means 

 that the total number of bands in the whole spectrum must 

 be considerably reduced — to say 5 or 10, and the thickness 

 of the mica plate must be correspondingly diminished. This 

 in turn involves the reduction of the aperture of the beam 

 which beyond a certain limit becomes inconvenient. But 

 the relation between the width of the diffraction pattern and 

 the dispersion of the prism can also be adjusted by reducing 

 the latter instead of increasing the former by making the 

 aperture narrow. A very suitable arrangement for this 

 purpose was found in the employment of an achromatic 

 combination of two prisms which could be made to give the 

 small dispersion necessary by simply altering the inclination 

 of one to the other. Fairly wide apertures could now be 

 used, and the process of adjustment for best visibility became 

 extremely simple, consisting not in the somewhat inconvenient 

 adjustment of the width of the aperture bisected by the edge 

 of the mica, but in the comparatively easy alteration of the 

 dispersion by changing the relative inclination of the two 

 prisms. A Nicol prism was also unnecessary with the thin 

 mica plates now employed. The method was quite successful 

 even with mica giving a retardation of about 5 or 6 wave- 

 lengths in the yellow region of the spectrum, corresponding 

 to only three Talbot's bands in the entire visible spectrum. 



With about the best adjustment, the uniformity of the 

 colour within a band and the sudden change in passing from 

 one band to another was strikingly evident. The colour of 

 the bands in a few typical cases is given below : — 



5 Bands. 



1. Red. Orange. Green. Blue. Violet. 



2. Scarlet. Yellow. Bluish green. Blue. Violet. 



3 Bands. 

 Orange. Blue. Violet. 



It will be seen that this method easily lends itself to the 

 study of the colours at the various stages of adjustment, and 

 it is very instructive to watch the gradual disappearance of 



