298 Determination of the Laws according to which Light 



nets solely on the Imnineus molecules which compose the peculiar 

 tint of tlie body. The first, under a certain incidence, polarizes 

 ill a great measure the light in the direction of the plane of re- 

 flection, after the manner of diaphanous bodies ; the second, on 

 t!ic . contrary, docs not produce this efiect, or at least produces 

 it witli a nmch less intensity. Hence it is easy to conclude, 

 that if \vc place a glass so that it may transmit or absorb the 

 first species of light, it M'ill reflect the otlier ; and we shall see 

 the body in its own colour, without any mixture of foreign white- 

 ness. Such, in fact, is the process which 1 have employed in 

 the work cited, in order to bring out the ]:)eculiar colours of gold, 

 iron, and copper. But I thought th.»n that the portion of light 

 of which these colours are composed issued from the bodies with 

 a polarization completely confused. M. Arago has hiformed 

 me that a very considerable portion issued from all sides, po- 

 larized parallel to the surface of the body and jjerpendicular to 

 the plane of emergence, winch, in fact, I have since had an op- 

 portunity of verifying, ^y e knew nothing more as to the inode 

 of polarization which the metals exercise, when Mr. Brewster 

 wrote me, that by reflecting several times on laminae of silver or 

 of gold, a streak of light already polarized, this light was so 

 modified, that on analysing it with a prism of Iceland spar it 

 was divided into two fasciculi differently coloured. I exerted 

 myself to verify this remarkable observation ; and the better to 

 distinguish the nature of the tints, I made to fall ujion the la- 

 minae a streak of white day light previously polarized in black 

 o-lass ; then by varying the incidences of the rays on the la- 

 ininte, it was easy to ascertain that the tints, into which the fas- 

 ciculus reflected was disidcd, were precisely those of the coloured 

 lings reflected and transmitted which were observed by Newton; 

 and that in this respect, as well as from the direction of the po- 

 larization, these phaenomena followed absolutely the laws of 

 nioveable polarization observed in the thin crystallized laminae. 

 I connnunicatcd this analogy to the Institute on the 27th of 

 March Tast, in giving an account of the new discovery of Mr. 

 Brewster, and I instantly communicated it to that gentleman 

 also. At the present moment multiplied observations put me 

 in a condrtion of establishing it with greater certainty, and of 

 proving that silver as well as the other metals ujodify the light 

 which they reflect, precisely as crystals endowed with double re- 

 fraction modify those which they refract ; the number of succes- 

 sive reflections answering to the greater or less thickness of the 

 t-rystal. — Such is the object of the paper I now present. 



But by an accident wliieh ought not to surprise in a field of 

 incpiiry so new, it happened that I at first observed different 

 phienomena, at least in appearance, from those seen by Mr. 



Brewster. 



