ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
521 
The author has applied the last formula to the calculation of the 
dimensions of the fibres. The section is mounted on a Babinet gonio- 
meter (fig. 52), and the angle 2 8 measured for sodium light by means of 
the telescope. 
Fig. 52. 
If a transverse section is taken instead of a tangential one, and a 
small circular aperture is observed across it instead of a narrow slit, 
phenomena of very brilliant coloured arcs are eshibited. The luminous 
circle is seen very clearly in the centre of the figure (0, fig. 53) ; on each 
Fig. 53. 
side, inclined in the direction of the layers of the wood, two luminous 
bands, which widen and become more coloured as they recede from the 
centre (L), are observed. Above and below these oblique bands, spectra 
