hy the aid of Polarized Light. 199 



in its natural condition, has the power of diverting transmitted 

 polarized light in determinate directions. As a special investiga- 

 tion, he devoted his attention especially to the examination of the 

 cells of the coats of bulbs, in which he observed that the hori- 

 zontal section of their wall is invisible on the dark field when 

 the walls are placed parallel with the plane of polarization of the 

 transmitted rays, while they are seen with more or less bright 

 illumination when they are placed obliquely towards this plane, 

 and appear in the brightest light when they are inclined at an 

 angle of 45° to this plane. He subjected the phsenomena ex- 

 hibited by starch -granules to elaborate investigation, and ex- 

 plained the analogy between them and cell-walls. He overlooked 

 the distinction which exists between these two classes of struc- 

 tures in their action upon polarized light. In nuclei, and in 

 the other granular structures occurring in cells, he likewise 

 detected traces of an influence exerted upon polarized light. 



Ehrenberg^s treatise differs essentially in character from that of 

 Erlach. While the latter worked more as a physicist, and endea- 

 voured to deduce by the aid of calculation, from a small number 

 of accurate observations, the laws according to which vegetable 

 matter acts upon polarized light, — Ehrenberg's essay is infinitely 

 richer in observations in all the kingdoms of nature, but on the 

 other hand, his theoretical reflections are less satisfactory. In 

 opposition to Erlach, Ehrenberg totally denies that all organic 

 tissues possess the power of doubly-refracting light ; in particular 

 he disputes the existence of this property, among the class of ob- 

 jects now under consideration, in the nucleus of the vegetable cell, 

 chlorophyll-granules, yeast, inulin, the tissue of fleshy and fila- 

 mentous Fungi, and in the scales upon the leaves of Olea, Rho- 

 dodendron, and Myrica. He gives special illustrations of the 

 scales of certain plants and of starch-granules. From the cir- 

 cumstance that the scales of Tillandsia usneoides and other plants, 

 similar in structure to those of the plants above named, act very 

 strongly upon polarized light, while the others do not exert this 

 influence, he drew the conclusion that the cause of the action 

 of those scales did not lie in the organic structure alone, and in 

 the arrangement of the membranous cells, but in some doubly- 

 refracting substance that was spread over the external or in- 

 ternal surface of the membrane of those cells ; this substance, 

 though it agreed with starch in the character that it could be 

 removed by acids, was not starch, because it could not be con- 

 verted into dextrine by heating. In reference to starch, Ehren- 

 berg remarks, that besides the ordinary roundish and longish 

 granules in which a rectangular or oblique cross is visible under 

 the polarizing microscope, there exists a third form (in Alpinia 

 Galanga and allied plants), where the granules are extended 



