PEOFESSOE STOKES ON THE LONG SPECTEUM OF ELECTETC LIGHT. 609 
Although the central part of the maxima of transparency in this figure is generally 
left white to save trouble, the reader must not suppose that that part of the spectrum 
suffers no absorption. On the contrary, it is more or less weakened when the solution 
has the strength to which the figure corresponds, and disappears altogether when the 
quantity of substance in solution is increased, while at the same time the edge of the 
first band of absorption creeps on a little towards the red, the absorption being usually 
pretty definite at this edge. The measurements were taken from the points where the 
light ceased to be sensible, which are represented in the figure by the junction of plain 
black and shaded white. The shading merely represents the general effect, the grada- 
tion of illumination not having been registered. It extends in the figure, as a general 
rule, too far to the left of the edge of the first black band, and accordingly does not 
represent the absorption at that limit as sufficiently definite. 
A glance at the figure will show how distinctive is the mode of absorption of the rays 
of high refrangibility by these different substances. Indeed this one character would 
serve to distinguish all these substances one from another, unless it be morphine 
from codeine, and caffeine from salicine. The dotted line in the figure for sesculine de- 
notes the commencement of the fluorescence, which is situated near the line G of the 
solar spectrum. A solution of brucine cuts off the invisible end of the solar spectrum 
about midway between the lines S and T, and accordingly not far from the end of the 
region which it requires a quartz prism and lens to see. Accordingly, when these sub- 
stances are examined by solar light their distinctive characters are almost wholly unper- 
ceived, the solutions of some appearing quite transparent, and those of others merely 
cutting off the extreme rays to a greater or less distance. With sesculine alone the 
maximum of opacity lies within the solar spectrum ; but even in this case we should have 
little idea of the great increase of transparency about to take place. 
The effect of acids and alkalies on all the glucosides referred to in the figure presents 
one uniform feature. When a previously neutral solution is rendered alkaline, the 
absorption begins somewhat earlier, when rendered acid somewhat later. With salicine 
there is merely an indication of this change, falling within the limits of errors of obser- 
vation ; but in the other cases it is quite perceptible, and with phlorizine the shifting of 
the band of absorption produced by an acid is very large. Fraxine (or paviine) agrees 
remarkably with sesculine in all its optical characters ; the maximum of absorption is 
merely situated a little nearer to the red, and the tint of the fluorescent light corresponds 
to a slightly lower mean refrangibility. 
Quinine presents no decided maximum of transparency. With this and the other 
bases observed, with one exception, the absorption, if changed at all, is changed in an 
opposite manner to the glucosides when the base is set free by ammonia. 
Bands of absorption occur also with neutral substances, for example coumarine and 
paranaphthaline, which last exhibits a system of such bands in the invisible part of the 
solar spectrum. 
Aconitine, atropine, and solanine exhibit no bands of absorption, but merely a general 
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