56 "N" RAYS 



Here is another verification of these results : 

 if for the fourth index the mean value 1*42 is 

 adopted, one works out that for an aluminium 

 prism of 60, the incidence giving the minimum 

 deviation is 45 19', and that this deviation is 

 30 38' ; the observed deviation was 31 10'. 

 With the same incidence, the calculated devia- 

 tion of the radiation, whose index is 1*67, is 

 57 42' ; the observed deviation was 56 30'. 



I now pass on to the determination of 

 wave-lengths. 



By means of the above-described arrange- 

 ment for studying dispersion by the prism of 

 27 15', refracted pencils are obtained, each of 

 which is sensibly homogeneous. If we make 

 the pencil we wish to study impinge on a second 

 screen of wet cardboard, pierced with a slit i '5 

 mm. wide, we can isolate a narrow portion of 

 this pencil. 



On the other hand, a piece of aluminium 

 foil is fixed to the moving radial arm of a 

 goniometer, so that its plane is normal to the 

 arm ; in this foil a slit is cut only O'OJ mm. 

 wide, and filled with phosphorescent calcium sul- 

 phide ; the goniometer is arranged so that its 



