520 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
colours can be clearly distinguished. (7) A black band, wide, but less 
intense than the preceding ones. (8) A very wide spectrum, of which 
about half the width is formed by the red colour. Beyond this spectrum 
comes a grey band of uniform colour. 
Analogous results, varying only in the disposition or number of the 
images observed, are obtained with other kinds of wood. A tangential 
section of Stillingia sebifera (fig. 51) gives two fine spectra on each side 
of the central slit, but less* wide than in the preceding case. 
Fig. 51. 
The t well-known formula giving the intensity I of a monochromatic 
light of wave-length A in a direction making an angle 8 with the direc- 
tion of the rays falling on a grating is : 
(i) i 
tt a sin 8 
! 77 a sin 8^ 2 
V A - ) 
. _ n 7r (a 4- b) sin 8 
sm 2 v - - 
. 0 7r (a 4- b) sin S’ 
sin 2 - 
in which a represents the constant width of the transparent parts left 
between the lines of the grating (corresponding to the lumen of the 
fibres), and b the width of the opaque lines (corresponding to the 
thickness of the walls of the fibres) ; n is the total number of lines of 
the gratings. 
The angle of deviation S of a monochromatic colour of wave-length A 
is given by the relation 
( 2 ) 8 = -^. 
a -f- 6 
m representing the number of order of the spectrum. Consequently the 
angle d enclosed between the directions of a simple colour belonging to 
two spectra of the same order, the one on the right and the other on 
the left of the central image, is given by 
(3) d = 28 = 
2mA 
a b 
