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THE ROYAL SOCIETY OF CANADA 



experiment is, moreover, extremely interesting in itself, involving a 

 series of fine optical adjustments and giving some very beautiful 

 results. 



Experimental Arrangement 



2. Fig. 1 shows the arrangement of apparatus. M is the movable 

 mirror of a Michelson Interferometer. The interferometer is accu- 



F/g. 2 



rately adjusted for a parallel "equivalent thin film," as shown by 

 circular fringes of apparent diameter independent of the position of 

 the observer. The two interferometer paths are accurately equal 

 when M occupies the position M^. M has been drawn up a suitable 

 distance, Z?/2 thereby introducing a difference of path D. The 

 system is illuminated by sunlight slightly converged by a condenser 

 lens so that the image of the sun will not be situated at the focal 

 planes of the lenses subsequently traversed by the light. 



G is the plane parallel glass plate of index /z whose dispersion is 

 to be investigated; it is placed in the adjustable path of the inter- 

 ferometer parallel to the mirror M so that it is traversed normally by 

 the rays normal to M. The index of refraction of G for the Dx line 

 as obtained by refractometer method is 1 . 57308 and the thickness of 

 the plate is . 9344 cm. (T/2). 



With this arrangement there exists a difference of path between 

 the two sets of interfering waves; the number of wave-lengths in this 

 difference varies throughout the spectrum not only on account of the 

 variation in the wave-length but also on account of the refraction and 



