by the aid of Polarized Light. 203 



If the Nicols be placed parallel, the neutral axes of the object 

 standing perpendicular to surfaces of the INicols, the light will 

 pass unaltered through the object and the upper Nicol j but when 

 the neutral axes are rotated under 45° in relation to the Nicol, 

 the light will suffer the above-described decomposition, and pass 

 through the upper Nicol with diminished intensity, and the body- 

 will consequently appear more or less dark in comparison with 

 the free part of the field. 



The foregoing phsenomena place us in a position to investigate 

 whether a body possesses a doubly-refracting power, and in what 

 direction its neutral axes lie. 



If we direct our attention to the vegetable organs above men- 

 tioned, it is clear, from their becoming visible in polarized light, 

 that their substance is doubly-refractive, and, from the position 

 of the four black lines, that one of the neutral axes lies in the 

 direction of the tangent of the circle, the other in the direction 

 of the radius. Since it is known that the membrane of these 

 annular structures is composed of concentric layers, the said 

 phsenomena indicate that in a lamellated cell-membrane viewed 

 in its transverse section, one neutral axis is parallel with the 

 lamellse, the other perpendicular to them. 



This is confirmed by the examination of any cellular tissue 

 whatever, in which it is found without exception, that those side- 

 walls of the cells which stand in a position perpendicular to one 

 of the two Nicols, are invisible, and that those membranes appear 

 in brightest illumination which are inclined at an angle of 45° 

 towards the Nicol. 



However, the phsenomena differ to a certain extent in thick- 

 walled and thin-walled cells. On the surface of a transverse 

 section of a thick-walled cellular tissue, for instance of the albu- 

 men of Phytelq)has, in which each individual cell, it is true, 

 represents in its extreme outline a quadrangle with straight 

 sides, but the secondary layers, in proportion as they lie more 

 internally, approach more and more the form of a circle by 

 the rounding-off of their angles, — it is impossible, from the 

 curved form of the majority of the layers, that any side-wall of 

 a cell can be placed perpendicularly to one of the Nicols in its 

 whole extent. Hence, in such a cell almost the whole of the 

 surface of the section will be visible in every position in polarized 

 light, and all that is seen is, as in the circular structure alread 

 mentioned, four black stripes, standing at right angles to each 

 other, but less regular in form and position than in the trans- 

 optical matters here concerned, and in particular as to the intensity of the 

 light dependent on the angle of inclination of the neutral axes to the 

 diameter of the Nicol, will find a minute account in the treatise of Erlach 

 above mentioned. 



