Interference-Bands 197 



plate, which should be mounted in such a fashion that the 

 pressure may be varied, and that the whole may be readily 

 turned in azimuth. The coloured bands are best seen when 

 the surfaces are nearly parallel and pretty close. It is best 

 to commence observations under these conditions. When 

 the achromatic azimuth has been found, the interval may be 

 increased. If it is desired to see a large number of bands, 

 a strip of paper may be interposed between the surfaces along 

 one edge, so as to form a plate of graduated thickness. 

 Talbot speaks of from 100 to 200 achromatic bands ; but I 

 do not think any such large number can be even approximately 

 achromatic. The composition of the light may be studied 

 with the aid of a pocket spectroscope, and the appearances 

 correspond with what has been already described under the 

 head of interference-bands formed from a prismatic spectrum 

 in place of the usual line of undecomposed light. As has 

 been already remarked, the colours of fine bands are difficult 

 to appreciate ; and indistinctness is liable to be attributed to 

 other causes when really due to insufficient achromatism. 



The use of a wedge-shaped layer of air is convenient in 

 order to obtain a simultaneous view of a large number of 

 bands ; but it must not be overlooked that it involves some 

 departure from theoretical simplicity. The proper develop- 

 ment of the light due to any thickness requires repeated 

 reflexions to and fro within the layer, and at a high degree of 

 obliquity this process occupies a considerable width. If the 

 band-interval be too small, complications necessarily ensue, 

 which are probably connected with the fact that the appearance 

 of the bands changes somewhat according to the distance from 

 the reflecting combination at which they are observed. 



Herschel's Bands. 

 In the system of bands above discussed, substantially identical 

 (I believe) with those observed by Talbot, all the rays of a 

 given colour are refracted under constant angles, the variable 

 element being the thickness of the plate of air. A system 

 in many respects quite distinct was described by W. Herschel, 

 and has recently been discussed by M. Mascart*. In this case 

 the combination of prism and plate remains as before, but the 

 thickness of the film of air is considered to be constant, the 

 alternations constituting the bands being dependent upon the 

 varying angles at which the light (even though of given 

 colour) is refracted. In order to see these bands all that is 

 necessary is to view a source of light presenting a large angle, 

 such as the sky, by reflexion in the layer of air. They are 

 * Loc. cit. ; also Traite d? Optique, torn. i. Paris, 1889. 

 Phil. Mag. S. 5. Vol. 28. No. 172. Sept. 1889. Q 



