70 JAMIN OX METALLIC REFLEXION, 



,, cos(l + 7) 1— tanitanr 



tan A'= rr r = r— — rr 



cos(l— r) l+tanitanr 

 1— tanA' . ,_o 



tan (45° -A') 

 tan? = 



tani 



(5.) 



Then, the azimuth of polarization of the incident light being 45°, 

 the incidence being i°, we shall measure A', calculate (r) by 



sin ^ 



sill * 



means of the formula (5.) and («) by the equation )i = ^t^. 



The value of (i) being arbitrary, we are at liberty to experiment 

 under various incidences, and to obtain from each experiment 

 values of {n) of which we shall take the mean. The following are 

 some of the results : — 



Incidences. Values of n. 



80 1-4909 



70 1-4932 



60 1-4896 



50 1-4949 



40 1-4900 



30 1-4965 



Mean . . 1-4925 

 This result differs only by three-hundredths from that given by 

 direct experiments for the index of refraction of glass : we shall 

 adopt it in the calculation of formuloe (l.),and the success of our 

 experiments wiU henceforth depend on the care with which the 

 angles (i) and {(3) are measured. I shall now enter into some 

 details on this subject. 



A horizontal circle having a copper stand, supports a tube 

 blackened in its interior, fixed on the circle, constantly directed 

 tov/ards the centre, and furnished at its two extremities with 

 cross-wires for the purpose of fixing the direction of the incident 

 ray. This tube carries a Nichol's prism which polarizes the 

 light, and whose direction is determined by a graduated vertical 

 circle placed on the tube. A second tube which receives the 

 reflected ray is moveable round the circle, and its displacements 

 are measured by means of a vernier ; the reflected light is ana- 

 lysed by a doubly-refracting prism placed at its exterior extre- 

 mitj^, and the direction of the principal section of this prism is 

 known bv means of a second vertical circle fixed on this move- 



