272 H. von Mohl on the Investigation of Vegetable Tissue 



Nicols, which has suffered no compression^ and which is so far 

 drawn out that the curves of the upper and under sides stand 

 about at right angles to each other, the greater part of the turns 

 are freely exposed to the light coming from the lower Nicol, and 

 the two layers of fibres only lie one over the other in triangular 

 spaces at the points of curvature of the fibrous bands at the 

 margins of the vessel. In such a vessel, as above remarked, 

 when a plate of selenite is interposed, the oppositely wound 

 fibres of the upper and lower sides appear in complementary 

 colours. These colours present themselves in great purity in all 

 parts where the ftbres lie free, but appear dulled in the triangles 

 above noticed. It here depends solely upon the focussing of the 

 microscope which colour we produce in these crossing-places : if 

 we focus carefully to the fibre of the upper wall, its colour (no 

 matter whether yellow or blue) will be almost pure, and little 

 affected by the colouring of the lower stratum of fibres, lying 

 under these circumstances out of focus ; on the other hand, if 

 we focus through the upper stratum of fibres to the lower, this 

 appears with its proper colour, without the upper stratum of 

 fibres, which cannot now produce any defined image on the 

 retina, producing any very great disturbance. Properly the 

 colours of the two layers of fibres should completely destroy 

 each other, and it cannot be doubted the light has undergone a 

 modification in passing through the inferior layer of fibres, and 

 again suffers it in the upper layer ; but with the isolated position 

 of the fibres produced by the longitudinal stretching of the ves- 

 sel, this light forms but an inconsiderable portion of the total 

 mass of light coming from the plate of selenite, and remains 

 almost ineffective after traversing the path from the lower to 

 the upper layer of fibres, so that no very striking modification 

 of the colouring of the upper layer of fibres can make itself felt. 



On the other hand, matters are totally changed when the 

 vessel is pressed flat in the compressor, so that the fibres of the 

 two layers come to lie one immediately above another at the 

 crossing-points, where the light modified by the lower fibre then 

 penetrates immediately into the upper fibre, without mixing with 

 the light penetrating unaltered beside the fibres of the lower 

 layer. Here the action of the two layers of fibres upon one an- 

 other is in most cases most evident, but it is indicated in very 

 various ways, according as the crossing of the fibres occurs or 

 not at a right angle. 



If the fibres cross at right angles, the effect upon polarized 

 light exerted by the lower stratum of fibres is removed by the 

 upper stratum, in the same way as happens with two plates of 

 mica crossing at right angles; hence the fibres should be in- 

 visible at the crossing-points. Now, this does not, it is true. 



