JAMIN ON METALLIC REFLEXION. 79 



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the values o' 7' 7;' u ^^ ^-'^^ difference of phase, and therefoi-e 



the corresponding angles of restored polarization ought to be 

 sensibly equal. We shall thus obtain numerous verifications. 



We perceive that it is sufficient to measure the incidence of 

 restored polarization ; as to the difference of phase, it is not 

 measured, but is known when the reflected ray has again become 

 polarized, and the number of reflexions has been counted. It 

 must also be remarked that the azimuth of polarization of the 

 incident ray is any whatever : the observed incidences do not 

 change when it vaines, and the polarizing prism of Nichol may 

 be placed as we please. If it be considered that it is always dif- 

 ficult to measure with precision the azimuth of the incident ray, 

 and that generally the slightest variation in its value alters the 

 results which are measured, some importance will be attached to 

 a process which leaves this quantity indeterminate, which re- 

 quires as an indispensable condition only the paralleUsm of the 

 plates, and which measures only one thing, the angle of incidence 

 of restored polarization. This practical simplicity will conduct 

 us to results of great accuracy. 



To obtain multiple reflexions, it is sufficient to place two mir- 

 rors of the substance to be examined parallel and opposite to 

 each other : light is to be made to fall on one of them, which 

 will be reflected from the second, come back on the first, &c. 

 The number of observable reflexions will evidently depend only 

 on the distance of the plates which ought to be variable at plea- 

 sure. The following is the an-angement which appeared to me 

 most commodious : — The two mirrors are fixed with wax on two 

 plates of brass, parallel and vertical ; the first is fixed, the second 

 is put in motion by a micrometer-screw, which transports it 

 parallel to itself. We may satisfy ourselves of the parallelism of 

 1! the mirrors by bringing them into contact, and noticing if all 

 i the edges accurately coincide. This little apparatus is placed 

 i on the centre of the graduated circle, of which I have already 

 I spoken : it is placed so that the polished surface of the fixed 

 I mirror passes through the centre of the circle. After having 

 I been reflected several times between the two mirrors, the ray 

 escapes into the air ; but then its direction prolonged, no longer 

 passes through the centre of the circle, and cannot traverse the 

 moveable tube in the direction of its axis. To remedy this in- 

 convenience, I caused the tube to have a horizontal movement 



