of Refraction of the Metals. 



11 



the metal prisms are brought into another medium, the de- 

 flection of the rays would alter — in correspondence with the 

 refractive index of the new medium. In order to try this, 

 experiments of the following kind were made. Upon the 

 glass plate carrying the prism a second glass plate was 

 cemented at a short distance from it, and the space between 

 was filled with a fluid, and the deflection was determined 

 again. If a! be the new deflection and n' the index of re- 

 fraction of the fluid with reference to air, and if a and 8 have 

 the same meaning as before, then, remembering that the 

 angle is small^ we have 



and 



a' = 8(n-n'), 

 a -a'=S(V-l). 



■We can thus calculate the deflection a! from the value of n' 

 determined in some other way, and from the difference 

 between a! and a the value of n' . 



The following small table contains the results of three ex- 

 periments. White light was used ; the value of n' being- 

 determined by means of a spectrometer for sodium light : — 



Metal prisms in Fluid. 



Observed. 



Calculated. 



8. 



a. 



«'. 



»', 



a! '. 



n'. 



2544 



Iron prism in oil of lav* 

 4-17-83 | 4-7-08 | 1-44 



nder. 

 | 4-8-64 



1-42 



855 



Copper prism in bone- oil. 

 -3-05 | -7-26 | 1-47 || -7-01 



1-49 



14-52 



Platinum prism in water. 

 4-9-61 | 4-4-36 | 133 || +4-78 



1-36 



The observed and calculated values agree very well 

 together. 



This appears to me to furnish the proof, so far as a purely 

 experimental proof is possible, that the above calculated values 

 of n really give the velocities of light in the metals. 



