H. von Mohl on the Investigation of Vegetable Tissue. 263 



XXIV. — On the Investigation of Vegetable Tissue by the aid of 

 Polarized Light, By H. von Mohl. 



[Concluded from p. 209.] 



A NEW and surprising series of phsenomena is produced when 

 the polarized pencil of light is made to pass through a doubly- 

 refractive medium, for instance, a thin plate of gypsum, mica, or 

 rock-crystal*, &c., in its way from the lower Nicol to the ob- 

 ject to be investigated. If these plates are so placed that their 

 neutral axes are directed obliquely to the Nicol (best at an angle 

 of 45°), the field appears more or less illuminated according to 

 the thickness of the plate used, and, if the plate has a certain 

 thickness, to be determined by trials, the field assumes one of 

 the colours of the Newtonian rings. The microscopic observa- 

 tion made by the help of this modified light is of a mixed kind, 

 because the object is, on the one hand, seen in transmitted light 

 as in the ordinary microscope ; on the other hand, this light is 

 on its own part again modified by the substance of the object, 

 and in consequence of this the object appears, as in the cases 

 above examined, as a self-luminous body, its various parts at the 

 same time presenting bright tints of complementary colours 

 according to their position in relation to the selenite-plate and 

 to the Nicols. It is well known that this arrangement is made 

 use of for the detection of weak degrees of doubly- refractive 

 power, since many objects which do not reveal this power by 

 mere application of two NicoPs prisms, are shown to be doubly- 

 refractive bodies on the interposition of such a plate, by their 

 peculiar colour differing from that of the field. It is not this 

 arrangement, however, which I here wish to discuss, but the 

 fact, hitherto overlooked, that there exist, in the behaviour of 

 vegetable membranes to polarized light, variations analogous to 

 those between positive and negative crystals, and that these 

 variations stand in connexion with chemical distinctions in the 

 vegetable organs. 



This phsenomenon may be observed either in annular struc- 

 tures or in transverse sections of smooth cell-membranes. Be- 

 tween the lower Nicol and the object is placed a plate of selenite, 

 which renders the field red; this plate is so rotated that its 

 neutral axes form an angle of 45° with the Nicols. If the micro- 

 scope is now focussed to an annular object, for example, the 

 cross-section of a cylindrical cell, this is seen divided into 

 four quadrants, which appear tinged brightly with complement- 

 ary colours in this way : the two alternate quadrants whose 



* I shall give details respecting the choice and application of these plates 

 in a future paper describing the arrangements of the polarizing microscope. 



