80 JAMIN ON METALLIC REFLEXION. 



of rotation round its support ; a direction may then be given to 

 it, in each case agreeing with that of the ray definitively deflected. 

 If the plates are sufficiently separated, we perceive the images 

 arising from one or two reflexions ; and when the mirrors are 

 brought near to each other, these images disappear : we see suc- 

 cessively those which arise from reflexions in greater number, 

 and can easily count them. 



The polarization is never perfectly restored v.-hen the incident 

 light is white. The inequality of action exerted by a metal on 

 the different simple rays of the spectrum renders the images 

 coloured, and we can only observe the incidence for which the 

 extraordinary image has the minimum of brightness ; but it is ob- 

 served that this minimum corresponds exactly to the intermediate 

 tint between the deep blue and dull purple. I contented myself 

 in the experiments on the silver plate with observing this inter- 

 mediate tint [teinte de passage), and taking for the angle of 

 restored polarization that for which tliis tint is a minimum in 

 the extraordinary image. Experiment moreover shows that it 

 varies in tint so rapidly with the direction of the principal sec- 

 tion of the analyser, and that it undergoes for the incidence 

 sought, so great a diminution of intensity, that the results lose 

 nothing of their explicitness, even when the number of reflexions 

 is very great. I have besides made observations with a red glass 

 on mirrors of steel, copper and zinc ; the results are represented 

 in the following tables : it will be observed that the differences 

 of phases follow exactly the law of variation which we have 

 already recognised in the oxides, and which has been previously 

 announced. 



