44 
PROFESSOR TYNDALL ON THE OPTICAL DEPORTMENT OP THE 
When the track of a parallel beam in dusty air is looked at horizontally through a 
Nicol’s prism, in a direction perpendicular to the beam, the longer diagonal of the 
prism being vertical, a portion of the light from the finer matter, being polarized, is 
extinguished. The coarser motes, on the other hand, which do not polarize the light, 
flash out with greater force, because of the increased darkness of the space around 
them. 
The individual particles of the finest floating matter of the air lie probably far beyond 
the reach of the microscope. At all events it is experimentally demonstrable that there 
are particles which act similarly upon light, and which are entirely ultra-microscopic. A 
few days ago, for example, an inverted bell-jar was filled with distilled water, into which, 
while it was briskly beaten by a glass rod, was dropped a solution of mastic in alcohol. 
The proportion was less than that employed by Brucke, being about 10 grains of the 
gum to 1000 grains of the alcohol. The jar was placed under a skylight, at the 
height of the eye above the floor. It was of a beautiful cerulean hue, this colour 
arising wholly from the light scattered by the mastic particles. Looked at horizontally 
through a Nicol’s prism, with its shorter diagonal vertical, the blue light passed 
freely to the eye. Turning the long diagonal vertical, the scattered light was wholly 
quenched, and the jar appeared as if filled with ordinary pure water. 
I tried the effect of a powerful filter upon those particles, and found that they passed 
sensibly unimpeded through forty layers of the best filtering-paper*. 
The liquid containing them was examined by a microscope magnifying 1200 dia- 
meters. The suspended mastic particles entirely eluded this power, the medium 
in which they swam being as uniform as distilled water in which no mastic whatever 
had been precipitated. 
The optical deportment of the floating matter of the air proves it to be composed, in 
part, of particles of this excessively minute character. The concentrated beam reveals 
them collectively, long after the microscope has ceased to distinguish them individually. 
They are, moreover, organic particles, which may be removed from the air by combustion. 
In presence of such facts, any argument against atmospheric germs, based upon their 
being beyond the reach of the microscope, loses all validity. 
We are here brought face to face with a question of extreme importance, which it 
will be useful to clear up. “ Potential germs ” and “ hypothetical germs ” have been 
spoken of with scorn, because the evidence of the microscope as to their existence was 
not forthcoming. Sagacious writers had drawn from their experiments the perfectly 
legitimate inference that in many cases the germs exist, though the microscope fails to 
reveal them. Such inferences, however, have been treated as the pure work of the 
imagination, resting, it was alleged, on no real basis of fact. But in the concentrated 
beam we possess what is virtually a new instrument, exceeding the microscope indefi- 
nitely in power. Directing it upon media which refuse to give the coarser instrument 
any information as to what they hold in suspension, these media declare themselves to 
* There are filters, however, which stop them ; but of this immediately. 
