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56 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 deviation 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 arrangement 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 1.5 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 0.07 mm. wide, and filled with phosphorescent calcium sulphide; the goniometer is arranged so that its