390 Astronomical and Nautical Collections. 



thicknesses of the plates of air which reflect similar tints in 

 the coloured rings. A short time after the publication of 

 the valuable memoirs of Mr. Biot on this subject. Dr. 

 Young remarked, that the difference of the paths of the 

 ordinary and extraordinary pencil, transmitted by a crystal- 

 lized plate, was precisely equal to that of the paths of the 

 rays reflected at the first and second surface of the plate of 

 air which affords the same colour ; and that this numerical 

 identity remained unchanged in every possible inclination of 

 the rays to the axis of the crystal. This very important 

 theoretical observation, which excited but little attention 

 when it was first published, served to give a new proof of 

 the fecundity of the principle of interference, as it established 

 the most intimate numerical relation between two classes of 

 phenomena, greatly differing from each other, as well in 

 the great disproportion of the thickness of the crystallized 

 plates, and the plates of air which exhibit the same colours, 

 as in the diversity of the circumstances necessary for their 

 production. 



Dr. YouNa demonstrated only by his calculations that the 

 colours of the crystallized plates were to be attributed to 

 the interference of the ordinary with the extraordinary un- 

 dulations : he did not attempt to explain in what circum- 

 stances this interference was possible, or why it was necessary 

 that the light should be polarised before its entrance into 

 the crystal, and should receive a new polarisation after its 

 emersion ; or how the intensity of the tints varied with the 

 relative direction of the primitive plane of the principal sec- 

 tion of the plate and that of the rhomboid. The principal 

 object of the memoir which I submitted to the Academy of 

 Sciences, the 7th of October, 1816, and of the supplement 

 added to it in the month of January, 1818, was to explain the 

 influence of these different circumstances, and to represent 

 the laws of the phenomenon by general formulas, which should 

 give for each image the intensity of the different kinds of 

 coloured ray. . I shall now explain this theory, continuing to 

 refer to experiment for the bases on which it is founded ; and 

 for the sake of greater simplicity, I shall suppose the light 

 to be homogeneous. 



