.TAMIN ON METALLIC REFLEXION. 71 



able tube. At the centre of the horizontal circle is a table, on 

 which the double plate is adjusted vertically in such a position, 

 that the line of separation of the two substances rests on the 

 exact centre of the apparatus ; this table is moveable round 

 the centre, and an " alidade," which traverses the limb of the 

 graduated circle, allows of the incidences being varied and 

 measured. 



The verticality of the double plate being an indispensable 

 condition, it was at first endeavoured to be accomplished by 

 known methods, and afterwards was verified by polarizing the 

 light in the principal azimuths, and observing that the polariza- 

 tion remained rectilinear after reflexion from the metal, and that 

 the azimuth was not altered by turning the reflecting surface 

 through 180°. In addition, two series of observations have 

 always been made, the reflecting surface being placed on the 

 right and left of the observer alternately, in order to correct 

 errors arising from a want of verticahty in the double plate. 



The incidences were measured, both by the deviation of the 

 reflected ray and by the displacement of the plate ; the angle 

 (/3) was ascertained with great precision ; in fact, the ease with 

 which the eye can judge of the equality of two lights of the 

 same tint is well known, and I found that a little practice 

 renders the sensibility of this organ truly remarkable. The re- 

 sults of experiments made under the same circumstances never 

 differ by more than fifteen minutes ; and if greater errors are 

 committed, it is because the points used for distinguishing 

 (jjoints de repere), whether for the measure of incidences or for 

 the position of the planes of polarization, are not always obtained 

 with so great an accuracy. Let it be observed, moreover, that 

 in each quadrant there are two angles (/3) and 90°— /3, which 

 render the ordinary or extraordinary image of the metal equal to 

 the extraordinary or ordinary image of the glass ; each result 

 therefore has been concluded from eight observations. 



In all my experiments the light was supplied by a Carcel 

 lamp, placed in a closed box at the focus of a lens which ren- 

 dered the rays parallel, so that the operations were conducted in 

 the most complete darkness : the light employed was very in- 

 tense, and always precisely the same ; it was made sensibly 

 homogeneous by a red glass of great thickness chosen with much 

 care, and which, whilst permitting the transmission of a sufficient 

 number of rays to render the obsei'vations easy, diminished the 



