hy the aid of Polarized Light. 201 



tion of regular circular structures, it being of little consequence 

 here whether we select the section of a colourless or only slightly 

 tinged * cylindrical cell, for instance, of a Nitella, or of a cylin- 

 drical vessel, for example, of the pitted tubes of a monocotyle- 

 donous plant, or the ring of an annular vessel. With the crossed 

 position of the NicoFs prisms, such an organ presents itself in 

 the black field of the microscope in the form of a ring, self-lumi- 

 nous with white light. But its light does not exhibit the same 

 intensity in all parts. In the direction of two lines crossing at 

 right angles in the centre of the ring and standing perpendicular 

 to the transverse diameter of the rhombic surfaces of the Nicol, 

 the ring is destitute of light, so that it is invisible at these places, 

 and appears divided by thin black stripes into four quadrants, 

 each of which exhibits the greatest illumination in the middle 

 (at a distance of 45° from the dark stripe), and becomes gradu- 

 ally duller outwards toward the black stripes. 



If the Nicols are placed parallel, the organ is seen on a 

 bright field, as in the ordinary microscope, like a transparent 

 body, but here again not uniformly bright, for now the pre- 

 viously black invisible parts of the ring transmit the brightest 

 light, while the formerly most clearly illuminated parts appear 

 more or less darkened. 



To pause a moment at this long-known fundamental phseno- 

 menon, and give an explanation of it, may not be superfluous to 

 many readers, and this is the more important, since the explana- 

 tion of most of the subsequent observations follows at once on 

 the comprehension of the facts above stated. 



The lower NicoFs prism, placed between the mirror and the 

 object, divides the light into two halves, which, according to the 

 now generally accepted undulatory theory, are so separated from 

 one another, that the plane in which the waves of one portion 

 vibrate, stands perpendicular to the plane of vibration of the 

 other portion. One of the halves of the light suffers total re- 

 flection by the Nicol, and hence does not arrive at the object ; 

 the other half, whose rays, as above said, all vibrate in one plane, 

 pass through the Nicol to the object and through this into the 

 microscope. This light however is incapable of passing through 

 the Nicol placed above the eye-piece, when this is made to cross 

 the lower Nicol at right angles ; therefore the field of the micro- 

 scope appears black. But these rays pass unobstructedly through 

 the second Nicol when it is placed parallel to the first, and hence 



* Dark colour of the membranes, as in many tropical woods, ferns, &e., 

 offers great obstacles to the detection of phajnomena of polarization. 

 When such organs are investigated, it is requisite to destroy the colour by 

 means of oxidizing agents, for instance, with Schulze's mixture of nitric 

 acid and chlorate of potash. 



