8 
DRS. J. PLUCKEE AND J. W. HITTORF ON THE 
increases to the eighth band ; over the ninth, tenth, and eleventh, especially over the 
two last, a shadow is spread, which gives to the red a rather brownish tint. The next 
seven bands are of a fine orange and yellow colour. The nineteenth and twentieth bands 
are very dark, the twenty-first is less dark. The following bands have a green colour, 
varying in brightness. The darkest are the twenty-eighth and twenty-ninth, succeeding 
the lightest ones. 
The cause producing these bands and their shading by dark transverse lines is 
evidently not the same as that which produces the shadow overspreading some of 
them. This may be concluded, for instance, from the fact that the shadow which 
darkens the nineteenth and twentieth bands, without entirely destroying their limits, 
spreads at the same time over the neighbouring third part of the preceding eighteenth 
band. 
18. When the light sent out from the incandescent nitrogen within the capillary 
tube is dispersed by means of four prisms, the shading of the less refracted bands also 
is resolved into dark narrow lines ; but these lines are smaller than the similar lines of 
the more refracted bands, and their distribution quite different. If the dispersion 
increase, in each band we at first perceive a new dark limit ; but the design becoming 
gradually more defined, we observe in each band extremely delicate bright lines 
bounded by a shadow or by dark lines. 
By closer examination of a band we distinguish first a least-refracted small part, 
occupying about the seventh part of the whole, formed by two bright lines including 
a somewhat larger dark space. The first of these two bright lines touches the dark 
extremity of the preceding band ; the second is bounded by a subtle dark line, to 
which succeeds a third bright line, smaller than the two first. A fourth bright line 
divides the whole band into two parts, one less refracted, comprising the small one just 
described, the other more refracted and larger — the breadth of the two parts being about 
in the ratio of 4 : 5. Starting from the bright middle line, a feeble shading is produced 
by a number of most subtle dark lines, the darkness of which decreases towards the 
least-refracted part. Similar but darker lines produce the stronger shading of the 
larger more refracted part, decreasing in the same direction from the extremity of the 
whole band towards its bright middle line. The stereoscopic effect produced by the 
shading of the bands is represented by the diagram (Plate I.). 
The configuration of all the bright orange and yellow bands is exactly the same ; it 
is rather obscured in the case of the preceding bands by the shadow spreading over 
them, but becomes the same again in the bright red ones. Even in the dark bands 19 
to 21, traces of the design are to be seen. The appearance of the green bands, though 
the general character be the same, slightly differs ; the shading in the middle part of 
them being increased, they rather seem to be divided into two. 
The accordance of these bands, even to the minute detail of their configuration, is a 
fact worthy of attention. 
19. The character of the two systems of bands on the extremities of the spectrum is 
