April 5, 1873.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
787 
through the one plate will be quenched by the other, a 
total interception of the light being the consequence. The 
experiment is now before you : the image of a plate of 
tourmaline is upon the screen. I place parallel to it 
another plate : the green of the crystal is a little deepened, 
nothing more. By means of an endless screw, I now turn 
one of the crystals gradually round ; as long as the two 
plates are oblique to each other a certain portion of light 
gets through ; but when they are at right angles to each 
other the space common to both is a space of darkness. 
It is on the green light which has been transmitted by 
the tourmaline that you are now to fix your attention, and 
now we may illustrate the two-sidedness of that green 
light. The light surrounding the green image being 
ordinary light is reflected in all directions; the green light, 
on the contrary, is not so reflected. The image of the 
tourmaline is now horizontal; I reflect it upward, and you 
see it still to be green. I reflect it sideways ; the image 
is reduced to blackness because of the incompetency of the 
green light to be reflected in this direction. Making the 
plate vertical and reflecting it as before, in the upper 
image the light is quenched ; in the side image you have 
now the green. Picture the thing clearly. In the one 
case the mirror receives the impact of the edges of the 
waves, and the green light is quenched. In the other case 
the sides of the waves strike the mirror, and the green 
light is reflected. To render the extinction complete, the 
light must be received upon the mirror at a special angle. 
What this angle is we shall learn presently. 
The quality of two-sidedness conferred upon light by 
Iceland spar may also be conferred upon it by ordinary 
reflection. Malus made this discovery in 1808, while look¬ 
ing through Iceland spar at the light of the sun reflected 
from the windows of the Luxembourg Palace in Paris. I 
receive upon a plate of window-glass the beam from our 
lamp ; a great portion of the light reflected from the glass 
is polarized ; the vibrations of this beam are executed, for 
the most part, parallel to the surface of the glass, and if I 
hold the glass so that the beam shall make an angle of 58 
degrees with the perpendicular to the glass, the whole of 
the reflected beam is polarized. It was at this angle that 
the image of the tourmaline was completely quenched in 
our former experiments. It is called the polarizing angle. 
And now let us try to make substantially the experi¬ 
ment of Malus. I receive the beam from the lamp upon 
this plate of glass and reflect it through the spar. Instead 
of two images, you see but one. So that the light, when 
polarized as it now is, can only get through the spar in 
one direction, and consequently produce but one image. 
Why is this ? In the Iceland spar, as in the tourmaline, 
all the vibrations of the ordinary light are reduced to two 
planes at right angles to each other; but, unlike the 
tourmaline, both beams are transmitted with equal facility 
by the spar. The two beams, in short, emergent from the 
spar, are polarized, their directions of vibration being at 
right angles to each other. When, therefore, the light 
was polarized by reflection, the direction of vibration in 
the spar which corresponded to the direction of vibration 
of the polarized beam transmitted it, and that direction 
only. But one image, therefore, was possible under the 
conditions. 
And now you have it in your power to check many of 
my statements, and you will observe that such logic as 
connects our experiments is simply a transcript of the 
logic of Nature. On the screen before you are the two 
disks of light produced by the double refraction of the 
spar. They are, as you know, two images of the aperture 
through which the light issues from the camera. Placing 
the tourmaline in front of the aperture, two images of the 
crystal will be obtained ; but now let us reason out what 
is to be expected from this experiment. The light emer¬ 
gent from the tourmaline is polarized. Placing the crystal 
with its axis horizontal, the vibrations of the transmitted 
light will be horizontal. Now the spar has two perpen¬ 
dicular directions of vibration, one of which at the present 
moment is vertical, the other horizontal. What are we to 
conclude ? Why, that the green light will be transmitted 
along the latter, which is parallel to the tourmaline, and 
not along the former, which is perpendicular to it. Hence 
we infer that one image of the tourmaline will show the 
ordinary green light of the crystal, while the other image 
will be black. Let us test our reasoning. You see it is 
verified to the letter. By means of an endless screw I can 
turn the crystal 90 degrees round. The black image 
becomes gradually brighter, and the bright one gradually 
darker ; at an angle of 45 degrees both images are equally 
bright; while, where 90 degrees have been obtained, the 
axis of the crystal being then vertical, the bright and 
black images have changed places. 
Given, two beams transmitted through Iceland spar, it 
is perfectly manifest that we have it in our power to de¬ 
termine instantly, by means of a plate of tourmaline, the 
directions in which the ether-particles vibrate in the two 
beams. I might place the double-refracting spar in any 
position whatever. A minute’s trial with the tourmaline 
would enable you to determine the position which yields 
a black and a bright image, and from these you would at 
once infer the directions of vibration. 
Further, the two beams from the spar being ' thus 
polarized, if they be received upon a plate of glass at the 
polarizing angle, one of them will be reflected, the other 
not. This is the conclusion of reason from our previous 
knowledge. 
I have said that the whole of the beam reflected from 
the glass at the polarizing angle is polarized; a word must 
now be added regarding the larger portion of the light 
which is transmitted by the glass. The transmitted beam 
contains a quantity of polarized light equal to that of the 
reflected beam ; but this quantity is only a fraction of the 
whole transmitted light. By taking two plates of glass 
instead of one, we augment the quantity of the transmitted 
polarized light; and by taking a bundle of plates, we so 
increase the quantity as to render the transmitted beam, 
for all practical purposes, perfectly polarized. Indeed, 
bundles of glass plates are often employed as a means of 
furnishing polarized light. 
Let us push our reasoning still further. When the 
tourmalines are crossed, the space where they cross each 
other is black. But we have seen that the least obliquity 
on the part of the crystals permits light to get through 
both. Now suppose, when the two plates are crossed, 
that we interpose a third plate of tourmaline between 
them, with its axis oblique to both. A portion of the 
light transmitted by the first plate will get through this 
intermediate one. But after it has got through, its plane 
of vibration is changed; it is no longer perpendicular to 
the axis of the crystal in front. Hence it will get through 
that crystal. Thus, by reasoning, we infer that the inter- 
po fition of a third plate of tourmaline will in part abolish 
the darkness produced by the crossing of the other two 
plates. I have not a third plate of tourmalinebut the 
talc or mica which you employ in your stoves is a more 
onvenient substance, which acts in the same way. Be¬ 
tween the crossed tourmalines, I introduce a film of this 
crystal. You see the edge of the film slowly descending, 
and as it descends between the tourmalines, light takes 
the place of darkness. The darkness, in fact, seems 
scraped away as if it were something material. 
We now stand upon the threshold of a new and splendid 
optical domain. We have to examine the chromatic 
phenomena produced by the action of crystals, and 
double-refracting bodies generally, upon polarized light. 
For a long time investigators were compelled to employ 
plates of tourmaline for this purpose, and the. progress 
that they made with so defective a means of inquiry is 
astonishing. But these men had their hearts in theii 
work, and were on this account enabled to extract great 
results from small instrumental appliances. But we 
have better apparatus now. You have seen the two 
beams emergent from Iceland spar, and have proved 
them to be polarized. If we could abolish one of these 
beams, we might employ the other for experiments on 
