Inj tlw uul of Pulitrizad Li[jht. 273 



occur quite strictly; but when the Nicols are used alone, the 

 crossing-points appear more or less black, while the rest ot" 

 the Hbres appear white ; and when the selcnite-plate is inter- 

 posed, the crossinfi;-|)oints exhibit the colour of the field, while 

 the uncrossed ptulions of the fibres are coloured yellow or blue, 

 according to their ])osition. For reasons readily perceived, the 

 want of colour at the crossing-points remains the same, however 

 the preparation may be rotated in a horizontal direction. 



But when the superposed fibres do not cross at right angles 

 (no matter whether more acutely or more obtusely), the pha:!no- 

 mena exhibited at the crossing-points are essentially different. 

 When a vessel of such kind stands perpendicular to one of the 

 Nicols, the right-ascending spiral will appear in the comple- 

 mentary colour of the left-ascending spiral, as in an unconi- 

 j)rcssed vessel, so far as the fibres do not overlie; but at the 

 crossing-points the lower stratum of fibres shows through the 

 upper with its ])roper colour (no matter whether blue or yellow) 

 almost unaltered, while the upper stratum is scarcely seen. 

 When, however, such a vessel is placed at an angle of 45° to the 

 Nicol, when the layers of fibres appear of the same colour, the 

 effects of the two fibres are added together at the crossing-points, 

 and at these places we find an analogous, but brighter, colour 

 than that of the free part of the fibi-es, — bright yellow instead of 

 dull yellow, bright blue instead of dull blue. 



In the spiral vessels prepared in the above manner, the fibres 

 of the upper and under sides act with equal force upon polarized 

 light on account of their equal thickness, and the course of the 

 fibres is perfectly regular : an equal regularity of the phenomena 

 there presented will not easily be found in the examination of 

 two adherent walls of adjacent cells, because here the unequal 

 thickness of the cell-wall and the irregular course of the fibres 

 diverted by the canals of the pits, produce disturbances, whence 

 the colouring of the cell-walls frequently becomes unequal, and 

 even complementary colours are found in neighbouring parts. 

 This is the case in a high degree in thick-walled cells of some- 

 what irregular form, as in liber-cells. 



Finally, it must be mentioned, in reference to the cell-wall 

 penetrated in a perpendicular direction by polarized light, that 

 in many cases, as already noticed by Schacht, the vicinity of a 

 round pit is distinguished by a black cross similar to that exhi- 

 bited by the cross-section of cylindrical cells. This cross is 

 most strikingly seen in the border which surrounds the pit of 

 Fir-wood ; it occurs, less sharply defined externally, on the pits 

 of the endosperm of Phytelephas, of Palms, and on many wood- 

 cells. The origin of this cross is easily explicable, from the fact 

 that the fibres of the membrane are diverted round the pits in 



Ann. ^ Mag. N. Hist. Ser. 3. Vol. i. 18 



