S 1 4 Imperial Institute of France. 



which are parallel or perpcr.dicular,\ve see, at every fourth 

 part of the revolution of the mica, the secondary images to 

 which this moveineiil gives rise, acquire an intensity eq'ial to 

 those preserved by the lirst imasrcs : if the plate interposed 

 becomes thinner, the number of rays which it depolarizes 

 also becomes smaller; so that, when a certain term is passed, 

 it acts like a common glass nnrror: the singular conse- 

 quence to which this experiment seems to lead, is, that 

 while the very thin plaies of mica have not the double re- 

 fraction, the plate which results from their superposition 

 enjoys this property in a very marked manner. 



Taking advantage of the properties which he had dis- 

 covered in refltcled light, M. Malus contrived a very sim- 

 ple apparatus, which is now usfd in the workshops where 

 prismatic micromelers are constructed, and by the help of 

 which vi'e may determine the axis of refraction and of cry- 

 stallization of bodies, and, whatever they mav be in other re- 

 spects, the alterations which they have undergone in their 

 external forms. In communicaiing the description of his in- 

 strument to the Class on the igth^of last August M. Malus 

 announced that he had applied it to the examination of 

 diaphanous mineral sut)stances, and of various chemical 

 products susceptible of crystallization, and had ascertained 

 that all these substances aie endowed with double refrac- 

 tion, except those which crystallize in the form of a cube, 

 or regular octahedron, by placing in similar circumstances 

 the fibrous and transparent parts of leaves and flowers, silk, 

 wool, or white hairs, fish scales, horn, ivory, quills, whale- 

 bone, &c. M. Malus further ascertained that all these 

 substances modify the lights in the same way as crystallized 

 bodies, so that all of ihcm have an axis of refraction, or of 

 crystallization, as if they were formed of molecules of a 

 determinate form arranged symmetrically with respect to 

 each other. 



In detailing these experiments, which are as new as they 

 are interesting, we have endeavoured to give them with 

 the greatest fidelitv, and in the precise terms used bv the 

 authors. We shall add to thern no observations of our 

 own : they are undoubtedly worthy, however, of the notice 

 of experimental philosophers, who by repeating them will 

 probably discover new phaenomena, or new applications of 

 those already known, and thus throw li^ht on some intri- 

 cate questions with which astronomers are perplexed. 



[To be continued.] 



LIII. In^ 



