806 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[April 12, 1873. 
ized ray where it enters the gypsum, is resolved into two 
ethers, vibrating at right angles to each other. 
In one of those directions of vibration the ether is more 
sluggish than in the other ; and, as a consequence, the 
waves that follow this direction are more retarded than the 
others. You can readily imagine that in this way the 
system of waves may get half a wave-length, or indeed any 
number of half wave-lengths, in advance of the other. 
The possibility of interference here flashes upon the mind. 
A little consideration, however, renders it evident that, as 
long as the vibrations are executed at right angles to each 
other, they cannot quench each other, no matter what the 
retardation may be. This brings us at once to the part 
played by the analyzer. Its sole function is to recompound 
the two vibrations emergent from the gypsum. It reduces 
them to a single plane, where, if one of them be retarded 
by the proper amount, extinction can occur. But here, as 
in the case of their films, the different lengths of the waves 
of light come into play. Red will require a greater thick¬ 
ness to produce the retardation necessary for extinction 
than blue; consequently, when the longer waves have been 
withdrawn by interference, the shorter ones remain and 
confer their colours on the film of gypsum. Conversely, 
when the shorter waves have been withdrawn, the thick¬ 
ness is such that the longer waves remain. An elementary 
consideration suffices to show that where the direction of 
vibration of prisms and gypsum incloses an angle of 45 
degrees, the colours are at their maximum brilliancy. 
When the film is turned from this direction, the colours 
gradually fade, until at the point where the directions of 
vibration of prisms and plate are parallel, they disappear 
altogether. 
A knowledge of these phenomena is best obtained by 
means of a model of wood or pasteboard representing the 
planes of vibration of the polarizer and analyzer, and the 
plane of vibration of the plate of gypsum between them. 
On these planes the waves may be drawn, showing the 
resolution of the first polarized ray into two others, and 
then the reduction of the two vibrations to a common 
plane. Following out rigidly the interaction of the two 
systems of waves, we are taught by such a model that all 
the phenomena of colours obtained when the planes of 
vibration of the two Nicols are parallel, are displaced by 
the complementary phenomena wdien the Nicols are per¬ 
pendicular to each other. 
In considering the next point, for the sake of simplicity, 
we will operate with monocromatic light—with red light, 
for example. Supposing that a certain thickness of the 
gypsum produces a retardation of half a wave-length, 
twice this thickness will produce a retardation of two half 
wave-lengths ; three times this thickness a retardation of 
three half wave-lengths, and so on. Now, where the 
Nicols are parallel, the retardation of half a wave-length, 
or of any odd number of half wave-lengths, with it pro¬ 
duces extinction ; in all thicknesses, on the other hand, 
which correspond to a retardation of an even number of 
half wave-lengths, the two beams support each other when 
they are brought to a common plane by the analyzer. 
Supposing, then, that we take a plate of dark ones. Here 
is a wedge-shaped film of crystal that shows these bands ; 
but they are far better shown by this circular film, which 
is so worked as to be thinnest at the centre, and gradu¬ 
ally increases in thickness from the centre outward. 
These splendid rings of light and darkness are thus pro¬ 
duced. 
Some of the chromatic effects of an irregular crystalli¬ 
zation are beautiful in the extreme. Could I introduce 
between our Nicols a pane of glass covered by those frost- 
ferns, which the cold weather renders now so frequent, 
rich colours would be the result. The cases of irregular 
crystallization on glass plates, now to be presented to you, 
illustrate what you might expect from the frosted window- 
pane. And not only do crystalline bodies act thus upon 
light, but almost all bodies that possess a definite struc¬ 
ture do the same. As a general rule, organic bodies act 
in this way; for this architecture involves an arrange¬ 
ment of the ether which involves double refraction. A 
film of horn, or the section of a shell, for example, yields 
very beautiful colours in polarized light. In a tree, the 
ether certainly possesses different degrees of elasticity, 
along and across the fibre ; and were wood transparent, 
this peculiarity of molecular structure would infallibly 
reveal itself by chromatic phenomena like those that you 
have seen. But not only do bodies built permanent by 
nature behave in this way, but it is possible to confer, by 
strain or by pressure, a temporary double-refracting struc¬ 
ture upon non-crystalline bodies, such as common glass. 
When I place this bar of wood across my knee and seek 
to break it, what is the mechanical condition of the bar ? 
It bends, and its convex surface is strained longitudinally ; 
its concave surface, that next my knee, is longitudinally 
pressed. Both in the strained portion and in the pressed 
portion the ether is thrown into a condition which would 
render the wood, were it transparent, double-refracting. 
Let us repeat the experiment with a bar of glass. Between 
the crossed Nicols I introduce such a bar. By the dim 
residue of light lingering upon the screen, you see the 
image of the glass, but it has no effect upon the light. I 
simply bend the glass bar with my finger and thumb, keep¬ 
ing its length oblique to the direction of vibration in the 
Nicols. Instantly light flashes out upon the screen. The 
two sides of the bar are illuminated, the edges most, for 
here the strain and pressure are greatest. In passing from 
strain to pressure, we cross a portion of the glass where 
neither is exerted. This is the so called neutral axis of the 
bar of glass, and along it you see a dark band, indicating 
that the glass along this axis exercises no action upon the 
light. By employing the force of a press, instead of the 
force of my finger and thumb, the brilliancy of the light is 
augmented. 
Again, I have here a square of glass which can be 
inserted into a press of another kind. Introducing the 
square between the prisms, its neutrality is declared ; but 
I can hardly hold it sufficiently loosely to prevent its action 
from manifesting itself. Already, though the pressure is 
infinitesimal, you see spots of light at the points where the 
press is in contact with the glass. I now turn this screen. 
Instantly the image of the square of glass flashes out upon 
the screen. You see luminous spaces separated from each 
other by dark bands. Every pair of adjacent luminous 
spaces is in opposite mechanical conditions. On one side 
of the dark band we have strain, on the other side pressure; 
while the dark band marks the neutral axis between both. 
I now tighten the vice, and you see colour ; tighten still 
more, and the colours appear as rich as those presented by 
crystals. Releasing the vice suddenly the colours vanish ; 
tightening suddenly, they reappear. From the colours of a 
soap-bubble Newton was able to infer the thickness of the 
bubble ; thus uniting by the bond of thought apparently 
incongruous things. From the colours here presented to 
you, the magnitude of the pressure employed might be 
inferred. Indeed, the late M. Wertheim of Paris invented 
an instrument for the determination of strains and pres¬ 
sures by the colours of polarized light, which exceeded in 
accuracy all other instruments of the kind. 
You know that bodies are expanded by heat and con¬ 
tracted by cold. If the heat be applied with perfect 
uniformity, no local strains or pressures come into play ; 
but, if one portion of a solid be heated and others not, the 
expansion of the heated portion introduces strains and 
pressures which reveal themselves under the scrutiny of 
polarized light. I place this square of common window- 
glass between the Nicols ; you see its dim outline, but it 
exerts no action on the polarized light. I hold it for a 
moment over the flame of a spirit-lamp ; on reintroducing 
it between the Nicols, light flashes out upon the screen. 
Here, as in the case of mechanical action, you have spaces 
of strains divided by neutral axes from spaces of pressure. 
Let us apply the heat more symmetrically. This small 
square of glass is perforated at the centre, and into the 
orifice a bit of copper wire is introduced. Placing the 
square between the prisms and heating the copper, the heat 
