Imperial Institute of France, 1 55 



fraction in some particular positions of the principal section 

 of the crystal to which we present it. 



The polarized rays differ from the rays of direct light by 

 several oilier properties which wereunkno^\n to Huyghens 

 and to Ne^vton, and the discovery of which is owing to 

 M. Mains. If we suppose m fact, that after having dis- 

 posed vertically the principal suction of a rhomboid ot car- 

 bonate ot 'inie, we receive the two tasciculi which [)rocted 

 from ihent on the surface of smooth x\ater, and under an 

 angle of 52" 45', we shall remark that the common fasci- 

 culus acts like the direct ray; since it abandons to the 

 partial reflection a pan of its molecules : as to the extra- 

 ordinary fasciculus, it penetrates the liquid quite through. 

 If we suppose, on the contrary, that the principal section 

 of the rhomboid is perpendicular to the plane ot incidence, 

 the extraordinarv ray underjioes the partial reflection, and 

 the ordinary ray penetrates the liquid entirely. 



When we examine, by the help of a rhomboid of cal- 

 careous spar, the light which is reflected on the surface of 

 the v\ater, and under an angle of 42" 45', we see that it has 

 all the characters of one of the fasciculi produced by the 

 double refraction of a crystal, for it no longer constantly 

 separates into two fasciculi: in this experiment, which 

 is in some measure the converse of that which we have 

 first referred to, the plane of reflection performs the office 

 of the principal section of the first rhomboid. We have 

 only explained these results (which are detailed at great 

 icnglh in the elegant work by M. Mains, to which the 

 Class decreed the jirize of Mathematics i'or IS 10) in order 

 to indicate the point from which the menibtvs of the Class 

 set out, who were occupied with this object in 181 1. 



We have hitherto only alluded to the modifications 

 which ihe luminous rays undergo in their reflection. Is the 

 light transmitted by diaphanous bodies, even modftjed in 

 certain circumstances which we are about to mention ? 



If we place two object glasses, one above the other, 

 there are formed, as v\e all know, some coloured ring?, 

 of which the point of contact is the common centre : 

 these rings are perceived either by the aid of icIlecteU 

 or of transmitted light. When the angle of the rays trans- 

 miited with the surface of the object-glass is about 32'\ 

 they are polarized, since in cerlain positions of the principal 

 section ot a crystal of Icelandic spar, we see only a single 

 image of rings. Now, il is a very remarkable circum- 

 »ianee in this experiment, that the moJifjcation which the 

 rays forming the rings undergo in passing through the 



object- 



