45-42 

 46-30 



10 Prof. A. Kundt on the Indices 



The silver double prism No. 2 of the above Table gave for 

 the sum of the angles of the prism in three observations the 

 values: — 



8*45) 

 10-26 VMean 9*68 divisions. 

 10-32 j 



The same converted into iodide of silver gave the angles 

 j Mean 45*86, 



which corresponds to an angle of 11*47 for the silver. 



So also a bismuth prism and a nickel prism, converted into 

 oxides, gave angles from which the angles of the metallic 

 prisms could be calculated with a sufficient agreement with 

 the angles directly measured. 



If the refracting angles are correctly measured by reflex- 

 ion, the observations of deviation can hardly be falsified by 

 variable change of phase. Nevertheless it might be supposed 

 that the diffraction of light might influence the observations 

 of deflection. The small aperture of the prism itself of course 

 produces a marked diffraction; and even with the best prisms 

 the image of the slit is not given with perfectly sharp edges. 

 Moreover the absorption of light by the prism increases 

 towards the base. I believe, therefore, that I have established, 

 by numerous experiments, that refraction produces no con- 

 stant error in the determinations of deflection. 



Further, the circumstance that prisms of very different 

 aperture, very different angles, and different thicknesses, in 

 which, consequently, the decrease in the intensity of light at 

 increasing distance from the edge of the prism was very dif- 

 ferent, gave values of a not affected by any regular change, 

 shows that diffraction could introduce into the observation no 

 error which would be greater than the general errors. 



Finally, we have the important question, What meaning is 

 to be given to the values of n calculated from the observa- 

 tions ? We see from the numbers given that n is a constant 

 for each metal and a given kind of light in the neighbour- 

 hood of the angle of prism employed, and does not depend in 

 any definite way on the value of these angles, which varied 

 in the case of iron and silver from one to four, n is of course 

 a magnitude characteristic of the optical behaviour of the 

 metal. If n, as in the phenomena of transparent media, is 

 the ratio of the velocity of light in the surrounding medium 

 (in our experiments, air) to its velocity in the metal, then, if 



